Intermediate Hybrids Research Articles

The genomic signatures of evolutionary stasis.

Evolutionary stasis characterizes lineages that seldom speciate and show little phenotypic change over long stretches of geological time. Although lineages that appear to exhibit evolutionary stasis are often called living fossils, no single mechanism is thought to be responsible for their slow rates of morphological evolution and low species diversity. Some analyses of molecular evolutionary rates in a handful of living fossil lineages have indicated that these clades exhibit slow rates of genomic change. Here, we investigate mechanisms of evolutionary stasis using a dataset of 1,105 exons for 481 vertebrate species. We demonstrate that two ancient clades of ray-finned fishes classically called living fossils, gars and sturgeons, exhibit the lowest rates of molecular substitution in protein-coding genes among all jawed vertebrates. Comparably low rates of evolution are observed at fourfold degenerate sites in gars and sturgeons, implying a mechanism of stasis decoupled from selection that we speculate is linked to a highly effective DNA repair apparatus. We show that two gar species last sharing common ancestry over 100 million years ago produce morphologically intermediate and fertile hybrids in the wild. This makes gars the oldest naturally hybridizing divergence among eukaryotes and supports a theoretical prediction that slow rates of nucleotide substitution across the genome slow the accumulation of genetic incompatibilities, enabling hybridization across deeply divergent lineages and slowing the rate of speciation over geological timescales. Our results help establish molecular stasis as a barrier to speciation and phenotypic innovation and provide a mechanism to explain the low species diversity in living fossil lineages.

Genomics-based assembly of a sorghum bicolor (L.) moench core collection in the Uganda national genebank as a genetic resource for sustainable sorghum breeding

The Uganda National GeneBank is a key reservoir of genetic diversity for sorghum (Sorghum bicolor (L.) Moench), with over 3333 accessions which are predominantly landraces (96.48%), but also includes the weedy accessions (0.63%), breeding lines (2.5%) and released varieties (0.39%). This genetic resource from the primary center of sorghum diversity and domestication is important for broadening the genetic diversity of elite cultivars through breeding. However, due to the large size of the collection, we aimed to select a core set that captures the maximum genetic and phenotypic diversity, in order to facilitate detailed genetic and phenotypic evaluation at a reduced cost. To achieve this, we genotyped the entire collection in 2020 using Diversity Array Technology sequencing (DArTseq). A total of 27,560 SNPs were used to select a core collection of 310 accessions using the GenoCore software. A comparison of core set and the whole collection based on the polymorphism information content, observed heterozygosity, expected heterozygosity and minor allele frequency showed no significant difference between the two sets, indicating that the core collection adequately captures the genetic diversity and allelic richness present in the whole collection. The core collection captures all the five major sorghum races and the 10 intermediate hybrids. The most strongly represented race is guinea (24.5%), while caudatum-bicolor is least frequent (0.69%). Landraces account for 92.2% of the core collection, whereas breeder’s lines, weedy accessions and released varieties contribute 2.2%, 3.5% and 1.9%, respectively.

Mathematical models to estimate forage production in southeastern Coahuila Mexico

Objective: Calibrate two non-linear models, in three intermediate triple hybrids, by theoretically comparing the accumulation of dry matter in relation to the days after sowing.
 Methodology: The cuts were every 14 days, from 30 to 170 das, and were adjusted to the Logistic and Richards models. The experimental design was a randomized block, with three replications.
 Results: The models explained most of the total dry matter yield in corn, observed in the field at 83%. The best fit model was the logistic model in cultivar AN447 and the Richards model in cultivar A7573, both with R2=0.98. The maximum yield simulated with the Richards model was shown in AN447 with 22,616 kg DM ha-1 and the lowest in AN388 with 10,970 kg DM ha-1.
 Limitations/Implications: Tests of other models in the same conditions and in other environmental conditions with the models under study.
 Conclusion: The Logistic model allows to simulate with greater precision the yield of dry matter in corn, by using the days after planting as an independent variable.

Functional genome-wide short hairpin RNA library screening identifies key molecules for extracellular vesicle secretion from microglia

SUMMARYActivated microglia release extracellular vesicles (EVs) as modulators of brain homeostasis and innate immunity. However, the molecules critical for regulating EV production from microglia are poorly understood. Here we establish a murine microglial cell model to monitor EV secretion by measuring the fluorescence signal of tdTomato, which is linked to tetraspanin CD63. Stimulation of tdTomato+ cells with ATP induces rapid secretion of EVs and a reduction in cellular tdTomato intensity, reflecting EV secretion. We generate a GFP+ tdTomato+ cell library expressing TurboGFP and barcoded short hairpin RNAs for genome-wide screening using next-generation sequencing. We identify Mcfd2, Sepp1, and Sdc1 as critical regulators of ATP-induced EV secretion from murine microglia. Small interfering RNA (siRNA-based) silencing of each of these genes suppresses lipopolysaccharide- and ATP-induced inflammasome activation, as determined by interleukin-1β release from primary cultured murine microglia. These molecules are critical for microglial EV secretion and are potential therapeutic targets for neuroinflammatory disorders.

Candidate-species delimitation in Desmognathus salamanders reveals gene flow across lineage boundaries, confounding phylogenetic estimation and clarifying hybrid zones.

Dusky Salamanders (genus Desmognathus) currently comprise only 22 described, extant species. However, recent mitochondrial and nuclear estimates indicate the presence of up to 49 candidate species based on ecogeographic sampling. Previous studies also suggest a complex history of hybridization between these lineages. Studies in other groups suggest that disregarding admixture may affect both phylogenetic inference and clustering‐based species delimitation. With a dataset comprising 233 Anchored Hybrid Enrichment (AHE) loci sequenced for 896 Desmognathus specimens from all 49 candidate species, we test three hypotheses regarding (i) species‐level diversity, (ii) hybridization and admixture, and (iii) misleading phylogenetic inference. Using phylogenetic and population‐clustering analyses considering gene flow, we find support for at least 47 candidate species in the phylogenomic dataset, some of which are newly characterized here while others represent combinations of previously named lineages that are collapsed in the current dataset. Within these, we observe significant phylogeographic structure, with up to 64 total geographic genetic lineages, many of which hybridize either narrowly at contact zones or extensively across ecological gradients. We find strong support for both recent admixture between terminal lineages and ancient hybridization across internal branches. This signal appears to distort concatenated phylogenetic inference, wherein more heavily admixed terminal specimens occupy apparently artifactual early‐diverging topological positions, occasionally to the extent of forming false clades of intermediate hybrids. Additional geographic and genetic sampling and more robust computational approaches will be needed to clarify taxonomy, and to reconstruct a network topology to display evolutionary relationships in a manner that is consistent with their complex history of reticulation.

Molecular Insight into Genetic Structure and Diversity of Putative Hybrid Swarms of Pinus sylvestris × P. mugo in Slovakia

The genetic structures of the four putative hybrid swarms of Pinus sylvestris × P. mugo in Slovakia were analyzed in terms of individual admixture proportions calculated via inter-primer binding site (iPBS) marker loci. This work aimed to reevaluate the hybrid swarms’ differentiation status as postulated in the previous studies at both population and genomic levels. The study confirmed intermediate divergence of each of the swarms examined. Based on 80-loci hybrid index scores, we have revealed the presence of introgressive and intermediate hybrids with frequencies corresponding to differentiation estimates. Surprisingly, irrespective of individual phenotypes, the most frequent intermediates were found in Sucha Hora (29.5%) and Obsivanka (28.6%) populations, which resemble rather pure P. mugo and were previously considered as bimodal hybrid zones with a negligible amount of hybrids. The remaining hybrid zone population Zuberec seems to be highly introgressed to P. sylvestris, while Tisovnica is clearly inclined to P. mugo. The results and different methodologies are discussed.

TECHNICAL PROPOSAL FOR MODIFICATION OF MAIZE SOWING DATE IN THE FUERTE-MAYO REGION, SONORA, MEXICO

Maize is the second most important crop in southern Sonora, Mexico, so it is an asset for the economy of the region, and it is highly relevant for the fall-winter crop pattern. The Fuerte-Mayo region is conducive for maize high grain yield potential; however, farmers used to establish the crop in a period of high frost occurrence. Therefore, the objective of this work was adjust the sowing date in order to optimize grain yield per hectare and escape the frost and heat risks. Experiments were established in the Fuerte-Mayo region which is located in the southwestern part of Sonora, during the crop season 2016-2017. Five commercial fields sown on different dates and in different areas of the region were selected for a follow up of the different phenological stages. Climatic information was taken close to the crop, by placing digital sensors within three of the fields; also, temperature data was taken from the automated weather stations from Module 01 and Zapata which belong to the weather network of the National Water Commission (CONAGUA), from October to June during the years 2015 to 2017, in order to identify months with high probability of frost and heat risks. Based on the temperature recorded by the digital sensors and the development of maize in those fields, three climatic zones were detected for avoiding the possible frost and heat risks: from November 15 to December 15 for early hybrids in a cold zone (central and northwest of the region), intermediate hybrids in the intermediate zone (central and east), and late hybrids in the warm zone (southeast and west). From November 15 to 30 for intermediate hybrids in the cold zone, and late hybrids in the intermediate zone, and from November 15 to December 31 for early hybrids in the intermediate and warm zones, and intermediate hybrids in the warm zone.

Two new hybrid populations expand the swordtail hybridization model system.

Natural hybridization events provide unique windows into the barriers that keep species apart as well as the consequences of their breakdown. Here, we characterize hybrid populations formed between the northern swordtail fish Xiphophorus cortezi and Xiphophorus birchmanni from collection sites on two rivers. We use simulations and new genetic reference panels to develop sensitive and accurate local ancestry calling in this novel system. Strikingly, we find that hybrid populations on both rivers consist of two genetically distinct subpopulations: a cluster of pure X. birchmanni individuals and one of phenotypically intermediate hybrids that derive ∼85-90% of their genome from X. cortezi. Simulations suggest that initial hybridization occurred ∼150 generations ago at both sites, with little evidence for contemporary gene flow between subpopulations. This population structure is consistent with strong assortative mating between individuals of similar ancestry. The patterns of population structure uncovered here mirror those seen in hybridization between X. birchmanni and its sister species, Xiphophorus malinche, indicating an important role for assortative mating in the evolution of hybrid populations. Future comparisons will provide a window into the shared mechanisms driving the outcomes of hybridization not only among independent hybridization events between the same species but also across distinct species pairs.

Population Genetics of a Translocated Population of Mottled Ducks and Allies

ABSTRACTTranslocating species is an important management tool to establish or expand the range of species. Success of translocations requires an understanding of potential consequences, including whether a sufficient number of individuals were used to minimize founder effects and if interspecific hybridization poses a threat. We provide an updated and comprehensive genetic assessment of a 1970s–1980s translocation and now established mottled duck (Anas fulvigula) population in South Carolina, USA. In addition to examining the population genetics of these mottled ducks, we simulated expected genetic assignments for generational hybrids (F1–F10), permitting formal purity assignment across samples to identify true hybrids and establish hybridization rates. In addition to wild mallards (A. platyrhynchos), we tested for presence of hybrids with migrant American black ducks (A. rubripes) and released domestic game‐farm mallards (A. p. domesticus). We used wild reference populations of North American mallard‐like ducks and sampled game‐farm mallards from 2 sites in South Carolina that could potentially interbreed with mottled ducks. Despite 2 different subspecies of mottled duck (Florida [A. f. fulvigula] and the Western Gulf Coast [A. f. maculatlus]) used in original translocations, we determined the gene pool of the Western Gulf Coast mottled duck was overwhelmingly represented in South Carolina's current population. We found no evidence of founder effects or inbreeding and concluded the original translocation of 1,285 mottled ducks was sufficient to maintain current genetic diversity. We identified 7 hybrids, including an F1 and 3 late‐staged (i.e., F2–F3 backcrosses) mottled duck × black duck hybrids, 1 F2‐mottled duck backcrossed with a wild mallard, and 2 F3‐mottled ducks introgressed with game‐farm mallard. We estimated a 15% hybridization rate in our mottled duck dataset; however, the general lack of F1 and intermediate hybrids were inconsistent with scenarios of high hybridization rates or presence of a hybrid swarm. Instead, our results suggested a scenario of infrequent interspecific hybridization between South Carolina's mottled ducks and congeners. We concluded that South Carolina's mottled duck population is sufficiently large now to absorb current hybridization rates because 85% of sampled mottled ducks were pure. These results demonstrate the importance in managing and maintaining large parental populations to counter hybridization. As such, future population management of mottled ducks in South Carolina will benefit from increased geographical and continued sampling to monitor hybridization rates with closely related congeners. We also suggest that any future translocations of mottled ducks to coastal South Carolina should originate from the Western Gulf Coast. © 2021 The Wildlife Society.

Expansion of the rare Eucalyptus risdonii under climate change through hybridization with a closely related species despite hybrid inferiority.

Hybridization is increasingly recognized as an integral part of the dynamics of species range expansion and contraction. Thus, it is important to understand the reproductive barriers between co-occurring species. Extending previous studies that argued that the rare Eucalyptus risdonii was expanding into the range of the surrounding E. amygdalina by both seed and pollen dispersal, we here investigate the long-term fitness of both species and their hybrids and whether expansion is continuing. We assessed the survival of phenotypes representing a continuum between the two pure species in a natural hybrid swarm after 29 years, along with seedling recruitment. The performance of pure species as well as of artificial and natural hybrids was also assessed over 28 years in a common garden trial. In the hybrid zone, E. amygdalina adults showed greater mortality than E. risdonii, and the current seedling cohort is still dominated by E. risdonii phenotypes. Morphologically intermediate individuals appeared to be the least fit. Similar results were observed after growing artificial first-generation and natural hybrids alongside pure species families in a common garden trial. Here, the survival, reproduction, health and growth of the intermediate hybrids were significantly less than those of either pure species, consistent with hybrid inferiority, although this did not manifest until later reproductive ages. Among the variable progeny of natural intermediate hybrids, the most E. risdonii-like phenotypes were the most fit. This study contributes to the increasing number of reports of hybrid inferiority in Eucalyptus, suggesting that post-zygotic barriers contribute to the maintenance of species integrity even between closely related species. However, with fitness rapidly recovered following backcrossing, it is argued that hybridization can still be an important evolutionary process, in the present case appearing to contribute to the range expansion of the rare E. risdonii in response to climate change.

Morphological and molecular characterization of Fasciola hepatica and Fasciola gigantica phenotypes from co-endemic localities in Mpumalanga and KwaZulu-Natal provinces of South Africa

Fasciolosis is a food- and water-borne disease caused by digenean trematode species, Fasciola hepatica and F. gigantica. They are widely distributed and infect a wide range of definitive hosts, causing enormous economic loss due to reduced productivity in domestic ruminants. The two species have been previously reported to be co-endemic in KwaZulu-Natal and Mpumalanga provinces of South Africa. Hybridization between the two species has been reported elsewhere. Despite the overlap of the two species in two provinces, there has been no attempt to determine the presence of the intermediate forms or hybrids. Therefore, this study aimed at morphological and molecular characterization of Fasciola spp. collected from cattle slaughtered at abattoirs located in the two provinces of South Africa, where two species are endemic. A total of 71 fluke specimens were collected cattle from abattoirs in Enhlazeni and Nelspruit in Mpumalanga province and Pietermaritzburg in KwaZulu-Natal province of South Africa, and Zimbabwe (Bulawayo abattoir). Fasciola gigantica specimen collected from Zimbabwe where it has been confirmed as the only species occurring and this was used as control in the morphological and molecular assessment of the collected specimens. Of the 71 specimens collected, 37 were classified as F. hepatica, 12 as F. gigantica and 22 as Fasciola spp using morphological characters. Of these species, 11 of 37F. hepatica and 6 of 22 Fasciola sp were found to be aspermic or having very scanty sperm. Fifteen flukes which were spermatic were all identified morphologically as F. gigantica whilst 5 flukes which were aspermic were identified morphologically as F. hepatica. Molecular analysis of the same 15 spermatic specimens confirmed the presence of F. hepatica (n=9) and F. gigantica (n=6) using the CO1 marker and as F. hepatica (n=4), F. gigantica (n=7) and Fasciola sp. (n=1) for the same specimens using the ITS-1/5.8S/ITS-2 marker. The remaining 4 aspermic flukes (one did not resolve) were all identified morphologically as F. hepatica and molecular analysis confirmed them as F. hepatica (n=4) by both CO1 and ITS-1/5.8S/ITS-2. Phylogenetic analysis based on both CO1 and ITS-1/5.8S/ITS-2 showed that F. hepatica species formed a moderately supported monophyletic clade with F. gigantica. Their ancestral history was further confirmed by haplotype network, which formed novel haplotypes that corresponded with the structure of the phylogenetic tree. Results from this study showed that morphological characters alone have limitations in identifying F. hepatica and F. gigantica in areas where the two species occur, although both methods confirmed the presence of F. gigantica occurring in Zimbabwe, F. hepatica in KwaZulu-Natal, and both species occurring in Mpumalanga province. Therefore, the use of morphological techniques, complemented by molecular techniques are recommended, especially in endemic areas where the two species are co-endemic.

Geochemical evolution of a composite pluton: insight from major and trace element chemistry of titanite

Titanite from various rocks of the Karkonosze granitoid pluton (South-Eastern Poland) was studied, in order to evaluate its precision in recording magma evolution processes. The rocks are of lamprophyric, dioritic, granodioritic and granitic composition, including hybrid structures such as microgranular magmatic enclaves and composite dykes. Based on textures, chemistry and Zr-in-titanite geothermometry, titanites can be divided into magmatic and post-magmatic populations. Late- to post-magmatic titanite is present in almost all rock types, especially in the most evolved ones (where magmatic titanite is absent) and can be characterized by low trace element and high Al and F contents. Magmatic titanite crystallized in temperatures between 610 and 870 °C, after apatite and relatively simultaneously with amphibole and zircon. Titanite from lamprophyre exhibits compositional features typical of titanites formed in mafic rocks: low Al and F, high Ti4+/(Al + Fe3+), LREE (light rare earth elemet)-enriched chondrite-normalized REE patterns, low Y/Zr, Nb/Zr, Lu/Hf, high (Ce + Nd)/Y, Th/U and Zr. Titanite from hybrid rocks inherited these characteristics, indicating major contribution of the mantle-derived magma especially during early stages of magmatic evolution. Titanite compositional variations, as well as a wide range of crystallization temperatures in hybrid granodiorites point to prolonged crystallization from distinct magma domains of variable mafic versus felsic melt proportions. The extent of compositional variations decreases through subsequent stages of magmatic evolution, and titanite with the least contribution of the mafic component is characterized by higher total REE, Al and F contents, lower Ti4+/(Al + Fe3+), (Ce + Nd)/Y and Th/U ratios, LREE-depleted chondrite-normalized REE patterns and higher Y/Zr, Nb/Zr and Lu/Hf ratios. Titanite composition from the intermediate and late stage hybrids bears signature of decreasing amount of the mafic melt and higher degree of its evolution, however, the exact distinction between the former and the latter is very limited.

What does a mixed population of Pinus sibirica and P. pumila from the southern Baikal region suggest about the structure of their hybrid zone?

Pinus sibirica and P. pumila have a vast hybrid zone, and its structure and maintenance model have not yet been considered. This study was carried out in the southern–western part of the hybrid zone. On the Khamar-Daban Ridge, there is a vigorous belt of mountain taiga forest, where the prevalent species is P. sibirica and where P. pumila produces a subalpine tree belt. The border between these belts is blurred, and a mixed zone of the species is formed, resulting in interspecies hybridization. Field observations, seed efficiency determination, and analysis of mt- and cpDNA markers were used to elucidate the pattern of this hybridization. Pinus sibirica and P. pumila hybridization was determined as bidirectional, and P. pumila was mainly the mother plant. Hybridization transforms to an introgression, and based on the life form, the studied hybrids were divided into two groups: intermediate and pumila-like cup-shaped forms. Both morphological types of hybrids had significantly fewer sound seeds per cone than the parental species. However, pumila-like hybrids had more sound seeds per cone compared to intermediate hybrids. We observed a positive correlation between hybrid seed efficiency and elevation. Summarizing the results of this study and those previously obtained enabled us to show different hybridization patterns in different parts of the hybrid zone, suggesting a mosaic model of hybrid zone maintenance.

Evaluation of Commercial Sorghum Hybrids for Resistance to Sugarcane Aphid, Melanaphis sacchari (Zehntner)

Studies in the field and growth chamber evaluated sorghum, Sorghum bicolor (L.) Moench, hybrids for resistance to sugarcane aphid, Melanaphis sacchari (Zehntner). Hybrids were known resistant ATx2752/RTx2783, known susceptible ATx2752/RTx430, and 14 with suspected resistance. The field study was done during 2015 and 2016 at three locations in Louisiana. The study was a split-plot design with hybrids randomized to main plots and insecticidal protection from aphid infestation (sprayed versus non-sprayed) randomized to sub-plots. Seedlings of the hybrids also were evaluated in a growth chamber. In the field, hybrids that seemed susceptible (demonstrated yield benefit after application of insecticide) were ATx2752/RTx430 and DG M756B39. Hybrids with possible tolerance (many cumulative aphid days but no yield benefit from insecticide) were DG M77GB52, DG 765B, and R94153. Hybrids with possible antibiosis (few cumulative aphid days and no benefit from insecticide) were ATx2752/RTx2783, RS84353, RS9813, SP6929, SP73B12, SP7715, SPX760, SP78M30, 83P17, W-844-E, and DKS37-07. Seedlings of ATx2752/RTx2783, DKS37-07, RS9813, RS84353, SP7715, SPX17414, SPX17514, NKX760, and W-844-E were significantly less damaged by aphids feeding than was ATx2752/RTx430. In addition to field and seedling studies, four hybrids were used to evaluate reproductive potential of sugarcane aphids in a growth chamber. Suspected resistant hybrid DKS37-07, suspected susceptible hybrid M75GB39, and two intermediate hybrids P83P17 and W -844-E were used. DKS3707 was the most resistant hybrid evaluated, with reduced intrinsic rate of increase of sugarcane aphids compared to the other hybrids. M75GB39 seemed the most susceptible of the three hybrids.

Variable hybridization outcomes in trout are predicted by historical fish stocking and environmental context.

Hybridization can profoundly affect the genomic composition and phenotypes of closely related species, and provides an opportunity to identify mechanisms that maintain reproductive isolation between species. Recent evidence suggests that hybridization outcomes within a species pair can vary across locations. However, we still do not know how variable outcomes of hybridization are across geographic replicates, and what mechanisms drive that variation. In this study, we described hybridization outcomes across 27 locations in the North Fork Shoshone River basin (Wyoming, USA) where native Yellowstone cutthroat trout and introduced rainbow trout co-occur. We used genomic data and hierarchical Bayesian models to precisely identify ancestry of hybrid individuals. Hybridization outcomes varied across locations. In some locations, only rainbow trout and advanced backcrossed hybrids towards rainbow trout were present, while trout in other locations had a broader range of ancestry, including both parental species and first-generation hybrids. Later-generation intermediate hybrids were rare relative to backcrossed hybrids and rainbow trout individuals. Using an individual-based simulation, we found that outcomes of hybridization in the North Fork Shoshone River basin deviate substantially from what we would expect under null expectations of random mating and no selection against hybrids. Since this deviation implies that some mechanisms of reproductive isolation function to maintain parental taxa and a diversity of hybrid types, we then modelled hybridization outcomes as a function of environmental variables and stocking history that are likely to affect prezygotic barriers to hybridization. Variables associated with history of fish stocking were the strongest predictors of hybridization outcomes, followed by environmental variables that might affect overlap in spawning time and location.

Additive Traits Lead to Feeding Advantage and Reproductive Isolation, Promoting Homoploid Hybrid Speciation.

Speciation through homoploid hybridization (HHS) is considered extremely rare in animals. This is mainly because the establishment of reproductive isolation as a product of hybridization is uncommon. Additionally, many traits are underpinned by polygeny and/or incomplete dominance, where the hybrid phenotype is an additive blend of parental characteristics. Phenotypically intermediate hybrids are usually at a fitness disadvantage compared with parental species and tend to vanish through backcrossing with parental population(s). It is therefore unknown whether the additive nature of hybrid traits in itself could lead successfully to HHS. Using a multi-marker genetic data set and a meta-analysis of diet and morphology, we investigated a potential case of HHS in the prions (Pachyptila spp.), seabirds distinguished by their bills, prey choice, and timing of breeding. Using approximate Bayesian computation, we show that the medium-billed Salvin’s prion (Pachyptila salvini) could be a hybrid between the narrow-billed Antarctic prion (Pachyptila desolata) and broad-billed prion (Pachyptila vittata). Remarkably, P. salvini’s intermediate bill width has given it a feeding advantage with respect to the other Pachyptila species, allowing it to consume a broader range of prey, potentially increasing its fitness. Available metadata showed that P. salvini is also intermediate in breeding phenology and, with no overlap in breeding times, it is effectively reproductively isolated from either parental species through allochrony. These results provide evidence for a case of HHS in nature, and show for the first time that additivity of divergent parental traits alone can lead directly to increased hybrid fitness and reproductive isolation.

Pattern of natural introgression in a Nothofagus hybrid zone from South American temperate forests

Interspecific gene flow is a common phenomenon in Nothofagaceae species; however, the dynamics of introgression in hybrid zones remains largely unknown. We focused on two ecologically and morphologically different Nothofagus species from Patagonia, Nothofagus nervosa and Nothofagus obliqua. In a natural hybrid zone, we established two plots 280 m apart in altitude (ca. 1.9 °C difference in mean temperature), and two subplots which captured microsite variation (abundance and spatial distribution of species and predominance of wind direction). We used intensive sampling of individuals (2055, including adults and regeneration) and molecular genotyping of 6 highly species-specific nuclear microsatellites for the identification and classification of hybrids, based on estimates of ancestry and interclass heterozygosity. We evaluated the relative contribution of our sampling effects to variation in hybrid incidence and direction of introgression using generalized linear mixed effects models. We determined that introgressive hybridization occurs at a global rate of 7.8% and that variation was mostly explained by plots (frequency at low altitude was approximately twice that found at high altitude), while it was less influenced by subplots. The high altitude plot was dominated by late-generation backcrosses to N. obliqua (asymmetric bimodality), whereas the low altitude plot consisted of intermediate hybrids (unimodality) and showed asymmetry for introgression between subplots. Differences were not detected between adults and regeneration, suggesting early-acting reproductive isolating barriers. F1 hybrids occur at a global frequency of 3.8%, and are fertile, as the detection of first- and late-generation hybrids indicates.

Metabolite profiling of Baltic blue mussel (Mytilus sp.) hybrids and their associated microbes

The blue mussel genus Mytilus is a popular food source and an important model organism for many scientific disciplines [1,2]. The morphologically similar species M. edulis (Me) and M. trossulus (Mt) form an unprecedented and unique hybridisation pattern in the Baltic Sea [3, 4]. Recently, it was shown that metabolite profiling of Mytilus species provides significant potential for biomonitoring of aquatic environments but also for informing natural product research [1,2]. However, the metabolome of Mytilus sp. is still little investigated and no metabolome data from the Baltic Sea blue mussels are available so far. In order to compare the metabolite profiles of Baltic Mytilus sp. at an individual level, 30 Mytilus specimens were sampled at a blue mussel farm in Kiel (Baltic Sea) and genotyped with two commonly utilized nuclear markers, EFbis and Glu-5' [4]. Genotypic assessment revealed a dominance of Mt alleles at Kiel Fjord with 90% of the samples being intermediate (Me/Mt) or Mt-like hybrids. Secondary metabolites of the whole tissue extracts (EtOAc) were explored by HPLC-DAD-MS. Metabolite profiles were found to be similar but not identical, differing mainly in minor nonpolar secondary metabolites. Considering that secondary metabolites are often produced by associated microbes and not by the macroorganism itself, the microbiome of the mussel specimens was also investigated. A culture-based approach yielded 259 bacterial and fungal strains, including Actinomycetes. Bacteria belonging to the orders Alteromonadales and Pseudomonadales (Acinetobacter sp., Pseudomonas sp., Pseudoalteromonas sp., Psychrobacter sp., Shewanella sp.) dominated the microbial community (57%). In addition, an RFLP based culture-independent approach was developed for Mytilus sp. individuals. Both the culture-dependent and culture-independent approaches showed inter-individual differences with respect to the associated microbes, but no genotypic dependence.

Population genomics of divergence within an obligate pollination mutualism: Selection maintains differences between Joshua tree species.

Speciation is a complex process that can be shaped by many factors, from geographic isolation to interspecific interactions. In Joshua trees, selection from pollinators on style length has been hypothesized to contribute to the maintenance of differentiation between two hybridizing sister species. We used population genomics approaches to measure the extent of genetic differentiation between these species, test whether selection maintains differences between them, and determine whether genetic variants associated with style length show signatures of selection. Using restriction-site-associated DNA (RAD)-sequencing, we identified 9516 single nucleotide polymorphisms (SNPs) across the Joshua tree genome. We characterized the genomic composition of trees in a narrow hybrid zone and used genomic scans to search for signatures of selection acting on these SNPs. We used a genome-wide association study to identify SNPs associated with variation in phenotypic traits, including style length, and asked whether those SNPs were overrepresented among the group under selection. The two species were highly genetically differentiated (FST = 0.25), and hybrids were relatively rare in the hybrid zone. Approximately 20% of SNPs showed evidence of selection maintaining divergence. While SNPs associated with style length were overrepresented among those under selection (P << 0.0001), the same was true for SNPs associated with highly differentiated vegetative traits. The two species of Joshua tree are clearly genetically distinct, and selection is maintaining differences between them. We found that loci associated with differentiated traits were likely to be under selection. However, many traits other than style length appeared to be under selection. Together with the dearth of intermediate hybrids, these findings reveal that these taxa are more strongly diverged than previously suspected and that selection, likely on many targets, is maintaining separation where the two species meet and hybridize.

Carbon Dioxide Enrichment Restrains the Impact of Drought on Three Maize Hybrids Differing in Water Stress Tolerance in Water Stressed Environments

Three maize genotypes were grown in controlled environment chambers with ambient (38 Pa) or elevated (70 Pa) carbon dioxide and water stress treatments were initiated 17 days after sowing. Shoot dry weight of the drought tolerant hybrid in both CO2 treatments was 44 to 73% less than that of the intermediate and sensitive hybrids when seedlings were well watered. Decreased shoot and root dry weights of the tolerant maize hybrid due to drought were about one-half that of the other two hybrids. Genotypic differences were observed in decreases of soil water content, leaf water potential, net photosynthesis and stomatal conductance in response to drought. Eleven of 19 amino acids measured in this study increased, methionine was unchanged and alanine and aspartate decreased in response to drought in the ambient CO2 treatment. Increased amino acid levels under elevated CO2 were observed at the end of the experiment. Significant genotypic differences were detected for amino acid responses to drought. Effects of drought on all three genotypes were mitigated by CO2 enrichment. Decreased shoot growth likely improved the stress tolerance of a highly drought resistant maize hybrid by reducing moisture loss, improving soil moisture content and increasing leaf water potential.