Chloroplast SSRs Research Articles

Genetic Diversity in Peripheral Pedunculate Oak (Quercus robur L.) Provenances—Potential Climate Change Mitigators in the Center of Distribution despite Challenges in Natural Populations

Croatian pedunculate oak (Quercus robur L.) populations represent southern range peripheral populations, often viewed as sources of valuable diversity and drought-resistant ecotypes. At the same time, they endure stronger selection pressures as a result of climate change. The leaves of 20 individuals per population (17) were sampled in a field trial and analyzed using 10 nuclear and 9 chloroplast SSRs to determine the level of intrapopulation genetic variability and genetic differentiation. Analysis with nSSRs revealed deviation from HWE in seven populations. AMOVA showed a high intra-population diversity (98.53%) and a small but statistically significant inter-population differentiation. Isolation by distance explained 19.6% of differentiation. Average FST between populations was low (0.013) compared with usual values for peripheral populations. Populations were rich in cpSSR haplotypes, confirming the hotspot of diversity caused by the encounter of recolonization routes. Unbiased haplotype diversity (HE) from 9 chloroplast SSRs and 325 individuals was (HE = 0.440). Sixty-six different haplotypes were grouped in three maternal lineages by both a median-joining network and a neighbor-joining algorithm. AMOVA for cpSSRs showed statistically significant diversity among populations (70.23%), suggesting genetic differentiation, but also a probable anthropogenic effect. AMOVA of nSSRs within and between lineages showed that original recolonization patterns of nuclear diversity were subsequently erased by gene flow.

First comparative analysis of complete chloroplast genomes among six Hedysarum (Fabaceae) species.

Hedysarum is one of the largest genera in the Fabaceae family, mainly distributed in the Northern Hemisphere. Despite numerous molecular studies on the genus Hedysarum, there is still a lack of research aimed at defining the specific characteristics of the chloroplast genome (cp genome) of the genus. Furthermore, the interrelationships between sections in the genus based on the cp genome have not yet been studied. In this study, comprehensive analyses of the complete cp genomes of six Hedysarum species, corresponding to sections Multicaulia, Hedysarum, and Stracheya were conducted. The complete cp genomes of H. drobovii, H. flavescens, and H. lehmannianum were sequenced for this study. The cp genomes of six Hedysarum species showed high similarity with regard to genome size (except for H. taipeicum), gene sequences, and gene classes, as well as the lacking IR region. The whole cp genomes of the six species were found to contain 110 genes ranging from 121,176 bp to 126,738 bp in length, including 76 protein-coding genes, 4 rRNA genes, and 30 tRNA genes. In addition, chloroplast SSRs and repetitive sequence regions were reported for each species. The six Hedysarum species shared 7 common SSRs and exhibited 14 unique SSRs. As well, three highly variable genes (clpP, accD, and atpF) with high Pi values were detected among protein-coding genes. Furthermore, we conducted phylogenetic analyses using the complete cp genomes and 76 protein-coding genes of 14 legume species, including the seven Hedysarum species. The results showed that the Hedysarum species form a monophyletic clade closely related to the genera Onobrychis and Alhagi. Furthermore, both of our phylogenetic reconstructions showed that section Stracheya is more closely related to section Hedysarum than to section Multicaulia. This study is the first comprehensive work to investigate the genome characteristics of the genus Hedysarum, which provides useful genetic information for further research on the genus, including evolutionary studies, phylogenetic relationships, population genetics, and species identification.

Establishing a baseline to monitor future climate-change-effects on peripheral populations of Abies alba (Mill.) in central Apennines

Understanding tree species responses to climate change is crucial for preserving biodiversity especially in Southern Europe hot spots where Abies alba is widely spread. Three Apennine silver fir populations, Pigelleto (PIG), La Verna (LV) and Bocca Trabaria (BT), ensured gene flows in interglacial periods between the two phylogenetically different groups of northern and southern Apennines. These stands were analysed (nuclear and chloroplast SSRs) with the aim to establish a baseline for their future management in view of the expected changes. The three forests were tested for the Centre-Periphery Hypothesis (CPH) compared to forty-five Italian populations. At the same time, permanent areas were surveyed within LV and PIG on dominant (a) and dominated or natural regeneration (r) tree layers, and on age classes. In two consecutive years, spring cambial phenology activity was also weekly monitored on microcores, and critical phenology dates recorded. The stands matched CPH only partially, showing different phylogenetic history and their bridging between northern and southern groups of silver fir populations was confirmed. LV was distinct from PIG and BT. The within-population variance component was significantly high, and no narrow relatedness was observed between dominant and dominated/regeneration spatially closer trees, and genetic parameters were comparable in both layers at LV and PIG. In both stands, older age classes ensured natural regeneration. Cambium phenology was highly variable within populations, consistently to other Mediterranean conifers, and highly sensitive to local and year’s conditions and monitoring will improve population’s adaptive capacity detection. Shelterwood-system silvicultural treatments are suggested on small areas to drive the demographic and panmictic balance towards an uneven-aged more resilient structure, and iterated monitoring will help to adapt the forest management to the isotherm shift.

New insights into the intraspecific cytoplasmic DNA diversity, maternal lineages classification and conservation issues of Tunisian pearl millet landraces.

Tunisian pearl millet (Pennisetum glaucum L.) landraces are still growing in contrasting agro-ecological environments and are considered potentially useful for national and international breeders. Despite its genetic potential, the cropping areas of this species are still limited and scattered which increases the risk of genetic erosion. The chloroplast DNA polymorphism and maternal lineages classification of forty nine pearl millet landraces representing seven populations covering the main distribution area of this crop in Tunisia were undertaken based on informative cpSSR molecular markers. A total of 21 alleles combining to 9 haplotypes were detected with a mean value of 3.5 alleles per locus and a haplotype genetic diversity (Hd) of 0.82. The number of chloroplast haplotypes per population ranged from 1 to 4 with an average of 1.28. The haplotypes median-joining network and UPGMA analyses revealed two probable ancestral maternal lineages with a differential pearl millet seed-exchange rate between the investigated areas. Northern and Central populations presented unique genetic backgrounds while historical farmers' practices in the South-East area resulted in the isolation of their own local landraces. The genetic evidences strongly support at least two introduction origins of pearl millet in Tunisia, one in the North and the other in the South followed by distinct local dispersal histories. Complementary in-situ and ex-situ conservation strategies taking into account the conservation of the maternal lineage cytoplasmic diversity are required. The investigated chloroplast SSRs provide useful molecular markers which could be used in further genetic studies and breeding surveys of pearl millet genetic resources.

The palaeoendemic conifer Pherosphaera hookeriana (Podocarpaceae) exhibits high genetic diversity despite Quaternary range contraction and post glacial bottlenecking

Glacial relict plants are often endangered because extant populations can be small, geographically isolated and persist in suboptimal environments, leading to increased clonality and reduced genetic diversity putting their survival at further risk. This study examines how restriction to interglacial refugia has impacted the genetic diversity and structure of the threatened Tasmanian palaeoendemic, Pherosphaera hookeriana W. Archer bis. This species is a poorly dispersed, dioecious conifer that, having once been a major component of Last Glacial vegetation, is now limited to 30 known populations. Genetic diversity and structure were assessed using fifteen nuclear and nine chloroplast SSRs in 23 populations representing the species’ entire range. Changes in distribution and abundance from the Last Glacial to present were investigated by examining the fossil record, approximate Bayesian computation (ABC) and species distribution modelling. Despite fossil and ABC based evidence for a postglacial bottleneck, species-level genetic diversity (He = 0.56 and Ne = 2.86) exceeded that of some conifers with far wider distributions. Significant genetic structure (Fst = 0.127, Jost’s D = 0.203) was present, with most populations dominated by distinct nuclear SSR genetic clusters and having unique chloroplast haplotypes. Unexpectedly, clonality plays only a small role in population level regeneration. Genetic diversity has likely been maintained due to dioecy, persistence in multiple parts of its range and extant populations being directly descended from proximate glacial populations. Protecting populations from the mounting threat of fire will remain crucial for the in situ conservation of P. hookeriana.

PCR analysis of experimental and commercial wines by means of nuclear and chloroplast SSRs

Genetic identification of varieties of grapevines in finished wines is still debated: several papers showed that DNA is extracted and analysed by PCR rather easily from the must, but few barely reproducible results have been presented for DNA extracted in wines after fermentation. This work experimented a method based on CTAB followed by silica purification with NucleoSpin Plant Kit columns to extract DNA from experimental wines of 1 year and commercial wines of 1 or 2 years. The comparison of SSR profiles of wines with those of their grapevine varieties showed that total identity was equal to 47.41% in experimental wines and to 24.31% in commercial wines. In experimental wines, three PCR replicates of three independent DNA preparations are sufficient to capture the alleles of the original grapevine variety, while in the commercial ones this possibility is related to the kind of wine and microsatellite.

Pollination dynamics in aPlatycladus orientalisseed orchard as revealed by partial pedigree reconstruction

Pollination dynamics was studied in a first-generation Platycladus orientalis (L.) Franco seed orchard with pedigree reconstruction using eight nuclear and four chloroplast SSRs. The pedigree reconstruction assigned 371 of 448 studied seeds to one of the orchard’s 192 candidate male parents and showed a high level of outcrossing and pollen contamination in the orchard’s seed crop. While the orchard’s seed population showed greater allelic richness compared with the parental population, a few alleles present in the parental population were missing in the seed crop. Additionally, we detected no significant correlation between male reproductive energy (pollen yield) and male reproductive success; however, uneven parental contribution was also observed. Pollen management practices were recommended to ensure the maintenance of genetic diversity in the seed crops and increase in genetic gain.

Development of high transferability cpSSR markers for individual identification and genetic investigation in Cupressaceae species.

Given the low substitution rate in plastomes, the polymorphic and codominant nature of chloroplast SSRs (cpSSRs) makes them ideal markers, complementing their nuclear counterpart. In Cupressaceae, cpSSRs are mostly paternally inherited, thus, they are useful in mating systems and pollen flow studies. Using e‐PCR, 92 SSR loci were identified across six Cupressaceae plastomes, and primers were designed for 26 loci with potential interspecific transferability. The 26 developed cpSSRs were polymorphic in four genera, Platycladus, Sabina, Juniperus, and Cupressus and are suitable for Cupressaceae molecular genetic studies and utilization. We genotyped 192 Platycladus orientalis samples from a core breeding population using 10 of the developed cpSSRs and 10 nuclear SSRs, and these individuals were identified with high confidence. The developed cpSSRs can be used in (1) a marker‐assisted breeding scheme, specifically when paternity identification is required, (2) population genetics investigations, and (3) biogeography of Cupressaceae and unraveling the genetic relationships between related species.

Ancient genetic bottleneck and Plio-Pleistocene climatic changes imprinted the phylobiogeography of European Black Pine populations

The historical changes in European Black Pine population size across the whole natural distribution in Europe and Asia Minor were analyzed facing the Plio-Pleistocene climatic fluctuations. Thirteen chloroplast SSRs and SNPs markers have been studied under the assumptions of “neutral evolution.” Populations and meta-populations had different histories of migration routes, and they were strongly affected by complex patterns of isolation, fragmentation, speciation, expansion (1.88–4.28 Ma), purification selection (2.09–21.41 Ma) and bottleneck (1.85–21.76 Ma). A significant number of populations (min. 29–41%) were in equilibrium for very long periods. Generally, the bottleneck revealed by chloroplast DNA is weaker than the bottleneck revealed by nuclear DNA. The N e immediately after the bottleneck reaches between 1820 and 3640 individuals. Generally, the historical effective population sizes shrink significantly for the Tertiary period from 10–15 up to 2.5 Ma in Western Europe (by 82%), followed by Asia Minor (69%) and the Balkan Peninsula (28%), likely resulting from important climatic changes. The rates and frequencies of stepwise westwards migration waves have been not sufficient to prevent isolation between the meta-populations and to suppress “sympatric speciation.” The migration was weak for the Pliocene, but was maximal for the Pleistocene, and finally silent for the present interglacial period, namely the Holocene.

Divergent Population Genetic Structure of the Endangered Helianthemum (Cistaceae) and Its Implication to Conservation in Northwestern China.

Population genetic studies provide a foundation for conservation planning, especially for endangered species. Three chloroplast SSRs (mtrnSf-trnGr, mtrnL2-trnF, and mtrnL5-trnL3) and the internal transcribed spacer were used to examine the population structure of Helianthemum in northwestern China. A total of 15 populations of the genus were collected. Nine chloroplast haplotypes and two nuclear genotypes were detected. Both the nuclear and chloroplast data showed two lineages in Helianthemum songaricum, respectively, distributed in Yili Valley and western Ordos Plateau. A total of 66.81% (p < 0.001) of the genetic variation was supported by this lineage split. A Mantel test showed a significant correlation between genetic distance and geographical distance (r = 0.937, p < 0.001). Based on genetic analyses, cpSSRs data support strong genetic divergence between regions. We speculate that the climate change during the late Tertiary and early Quaternary isolated H. songaricum into their current distribution, resulting in interruption of gene flow, leading to isolation and genetic divergence between the two regions. Meanwhile, possible selfing would increase genetic drift in small fragmented populations, that might account for the observed genetic divergence in both regions. Given the loss of genetic diversity and genetic divergence in small populations of Helianthemum in northwestern China immediate conservation management steps should be taken on the species.

Development and validation of the first SSR markers for Mimosa scabrella Benth.

Mimosa scabrella Benth., popularly known as ''bracatinga'', is a pioneer and endemic species of Brazil, occurring in Mixed Ombrophilous Forest associated with Brazilian Atlantic Rainforest biomes. It is a fast-growing tree of the Fabaceae family that facilitates the dynamics of ecological succession. SSR development, when there is no genome sequence, is time and labor intensive and there are no molecular markers for M. scabrella. We developed and validated the first microsatellite markers for this tetraploid species, evaluating mother trees and progenies. Using Illumina sequencing, we identified 290 SSR loci and 211 primer pairs. After 31 SSR loci PCR/agarose electrophoresis selection, a subset of 11 primer pairs was synthetized with fluorescence in the forward primer for PCR and capillary electrophoresis validation with leaf DNA of 33 adult and 411 progeny individuals. Polymorphic locus percentage was 36, 4 in 11 loci, 3 chloroplast SSRs, and 1 nuclear SSR. Allele number of polymorphic loci ranged from 2 to 11 alleles considering all sampling. All 11 primer pairs were also tested for cross-species amplification for five Fabaceae-Mimosoideae species, ranging from 2 loci transferred to Calliandra tweedii Benth. and all 11 loci transferred to Mimosa taimbensis Burkart. The assessed and validated SSR markers for M. scabrella are suitable and useful for analysis and population genetic studies.

Chloroplast Genome Sequence of Pigeonpea (Cajanus cajan (L.) Millspaugh) and Cajanus scarabaeoides (L.) Thouars: Genome Organization and Comparison with Other Legumes.

Pigeonpea (Cajanus cajan (L.) Millspaugh), a diploid (2n = 22) legume crop with a genome size of 852 Mbp, serves as an important source of human dietary protein especially in South East Asian and African regions. In this study, the draft chloroplast genomes of Cajanus cajan and Cajanus scarabaeoides (L.) Thouars were generated. Cajanus scarabaeoides is an important species of the Cajanus gene pool and has also been used for developing promising CMS system by different groups. A male sterile genotype harboring the C. scarabaeoides cytoplasm was used for sequencing the plastid genome. The cp genome of C. cajan is 152,242bp long, having a quadripartite structure with LSC of 83,455 bp and SSC of 17,871 bp separated by IRs of 25,398 bp. Similarly, the cp genome of C. scarabaeoides is 152,201bp long, having a quadripartite structure in which IRs of 25,402 bp length separates 83,423 bp of LSC and 17,854 bp of SSC. The pigeonpea cp genome contains 116 unique genes, including 30 tRNA, 4 rRNA, 78 predicted protein coding genes and 5 pseudogenes. A 50 kb inversion was observed in the LSC region of pigeonpea cp genome, consistent with other legumes. Comparison of cp genome with other legumes revealed the contraction of IR boundaries due to the absence of rps19 gene in the IR region. Chloroplast SSRs were mined and a total of 280 and 292 cpSSRs were identified in C. scarabaeoides and C. cajan respectively. RNA editing was observed at 37 sites in both C. scarabaeoides and C. cajan, with maximum occurrence in the ndh genes. The pigeonpea cp genome sequence would be beneficial in providing informative molecular markers which can be utilized for genetic diversity analysis and aid in understanding the plant systematics studies among major grain legumes.

Comparative assessment of genetic diversity in cytoplasmic and nuclear genome of upland cotton.

The importance of the cytoplasmic genome for many economically important traits is well documented in several crop species, including cotton. There is no report on application of cotton chloroplast specific SSR markers as a diagnostic tool to study genetic diversity among improved Upland cotton lines. The complete plastome sequence information in GenBank provided us an opportunity to report on 17 chloroplast specific SSR markers using a cost-effective data mining strategy. Here we report the comparative analysis of genetic diversity among a set of 42 improved Upland cotton lines using SSR markers specific to chloroplast and nuclear genome, respectively. Our results revealed that low to moderate level of genetic diversity existed in both nuclear and cytoplasm genome among this set of cotton lines. However, the specific estimation suggested that genetic diversity is lower in cytoplasmic genome compared to the nuclear genome among this set of Upland cotton lines. In summary, this research is important from several perspectives. We detected a set of cytoplasm genome specific SSR primer pairs by using a cost-effective data mining strategy. We reported for the first time the genetic diversity in the cytoplasmic genome within a set of improved Upland cotton accessions. Results revealed that the genetic diversity in cytoplasmic genome is narrow, compared to the nuclear genome within this set of Upland cotton accessions. Our results suggested that most of these polymorphic chloroplast SSRs would be a valuable complementary tool in addition to the nuclear SSR in the study of evolution, gene flow and genetic diversity in Upland cotton.

Population Structure of and Conservation Strategies for Wild Pyrus ussuriensis Maxim. in China.

Pyrus ussriensis Maxim. is native to the northern part of China, but whose habitats are currently being destroyed by environmental changes and human deforestation. An investigation of population structure and genetic diversity of wild Ussurian pear is a priority in order to acquire fundamental knowledge for conservation. A total of 153 individuals of wild Ussurian pear from the main habitats, Heilongjiang, Jilin, and Inner Mongolia in China, possessed low genetic diversity as a result of habitat fragmentation. The genetic diversity of the populations in Inner Mongolia and north east of Heilongjiang was especially low and there was the possibility of inbreeding. Wild Ussurian pears were divided into 5 groups based on the Bayesian clustering method using 20 nuclear SSRs (nSSRs) and 5 groups by haplotype distributions using 16 chloroplast SSRs (cpSSRs), and the populations in Inner Mongolia and north east of Heilongjiang represented unique genotypes. AMOVA indicated there was a 20.05% variation in nSSRs and a 44.40% variation in cpSSRs among populations. These values are relatively high when compared to those of other tree species. Haplotype E, positioned in the center of the cpSSR analysis network and showed the largest number of connections with other haplotypes, represented the most important haplotype. Inner Mongolia and the north east of Heilongjiang are two areas that need urgent conservation because of their genetic vulnerability and peculiarity. We determined 4 conservation units based on the clustering by nSSRs and cpSSRs, and geographic factor. This information is helpful in deciding the conservation strategies for wild Ussurian pear in China.

Association mapping for resin yield in Pinus roxburghii Sarg. using microsatellite markers

AbstractAssociation mapping is a method for detection of gene effects based on linkage disequilibrium (LD) that complements QTL analysis in the development of tools for molecular plant breeding. A total of 240 genotypes of Pinus roxburghii (Himalayan Chir Pine) from a natural population in Chakrata division (Tiunee range), Uttarakhand (India) were evaluated for resin yield. Based on the phenotypic data and stable resin production in consecutive years, 53 genotypes were selected after excluding the individuals with similar resin production. The selected 53 individuals were best representatives of the variation in resin yield in Chakrata population which varied between 0.25 and 8.0 kg/tree/year and were used for genotyping and association analysis using SSR markers. Out of 80 primers initially screened, a total of 19 polymorphic SSRs (11 cpSSR and 8 nSSR) were used in the study. Model based clustering using 19 polymorphic SSR markers identified five subpopulations among these genotypes. LD was evaluated using the entire population. The squared allele frequency correlation, r2 was estimated for each pair of SSR loci. The comparison wise significance (p-values) of SSR marker pairs was determined by performing 100,000 permutations. The genetic divergence ranged from 50 to 100%. The UPGMA based hierarchial clustering grouped the genotypes in accordance with their resin yield. Model based clustering suggested the existence of five subpopulations in the sample. However, the distribution of P. roxburghii genotypes into five subpopulations had no correlation with their resin yield thus ruling out the possibility of any ancestral relationship among the genotypes with similar resin yield. AMOVA suggested that the variation among P. roxburghii genotypes at the molecular level was related with the variation in resin yield and not their site of collection thus highlighting the genetic basis of the trait. LD based association analysis revealed two chloroplast SSRs Pt71936 and Pt87268 and one nuclear SSR pm09a to be in significant association with resin yield. The two associated chloroplast SSRs showed significant LD (p&lt;0.01). One of the chloroplast SSR Pt87268 showing association with resin yield was also found to be in significant LD with the nuclear SSR pm07, further showing the probability of this marker also to be associated with resin yield.

Morphological and genetic variation within and among four Quercus petraea and Q. robur natural populations

Numeric-taxonomical and molecular techniques [nuclear simple sequence repeats (nSSRs), expressed sequence tag (EST) SSRs, and chloroplast SSRs] were used to estimate morphological and genetic differentiation within and among 4 Quercus petraea and Q. robur natural populations in Turkey. Canonical discriminant analysis (CDA) and principal component analysis (PCA) showed that 3 out of 15 studied leaf morphological characters (clustered hairs, number of intercalary veins, petiole length) mainly accounted for the discrimination between species. In accordance with earlier observations, the mean genetic differentiation between Q. petraea and Q. robur at 8 microsatellite loci (F_{ST} = 0.030) and at chloroplast microsatellites (G_{ST} = 0.22) was low, potentially as the result of interspecific gene flow or shared ancestral polymorphisms. Chloroplast microsatellite analyses showed unique chloroplast haplotypes in Turkey, suggesting a glacial refuge in this region. Nuclear microsatellite ssrQrZAG96, described in other studies as being potentially under directional selection (outlier locus) due to high interspecific differentiation, showed considerably lower differentiation between species in the present study (F_{ST} = 0.028). These different patterns of genetic variation suggest different local adaptations of the species in this putative glacial refugial area in Turkey. It may also be that this marker is not directly associated with morphological or adaptive species differences. Interestingly, the analysed Q. petraea and Q. robur populations appear to have ecological requirements with regard to water availability that are different than those described for European populations. The analysis of additional populations within the species distribution range in Turkey is necessary to confirm this pattern.

Amazon diversification and cross‐Andean dispersal of the widespread Neotropical tree species Jacaranda copaia (Bignoniaceae)

AbstractAimThe phylogeographical history of Neotropical species can be difficult to reconstruct because of superimposed Neogene and Quaternary histories, and because of taxonomic uncertainty. We analysed range‐wide genetic diversity in a widespread pioneer tree species, Jacaranda copaia (Aubl.) D. Don, to characterize phylogeographical structure, date the evolutionary relationships among lineages, and evaluate the role of dispersal and vicariance in establishing the present geographical range.LocationGuiana Shield; central, southern and western Amazon Basin; Chocó region; Central America.MethodsWe analysed nine nuclear simple sequence repeat loci (nuSSR), eight chloroplast SSRs (cpSSR), and two cpDNA intergenic sequences in 341 adult trees. Genetic differentiation at nuSSRs was inferred using Bayesian clustering. Dating of chloroplast lineage divergence was obtained using a range of published mutation rates and Bayesian coalescence analyses. Population divergence dating was performed using an isolation‐with‐migration model for eight loci (one cp sequence and seven nuSSRs).ResultsNuclear SSR variation identified three geographically overlapping clusters (nu‐1, nu‐2, nu‐3). Twelve cpDNA haplotypes were clustered into two haplogroups (cp‐1, cp‐2) with the central Amazon harbouring the highest diversity. Molecular dating analysis suggests that cpDNA haplotype diversification started around the end of the Pliocene (2.61 Ma on average), whereas population divergence was more recent and occurred during the mid‐Quaternary (point estimates between 357 and 436 ka).Main conclusionsThe genetic variation of J. copaia in the Neotropics was shaped mainly by Pleistocene events. Chloroplast diversity did not display the expected cis/trans Andean disjunction, indicating recent dispersal. Nuclear variation revealed that separate regions share a recent history, with a centre of diversity in the central Amazon Basin. The geographical pattern of diversity is congruent with the distribution of the two subspecies, J. copaia copaia and J. copaia spectabilis, and evidence of nuSSR admixture between the two taxa supports their classification as subspecies.

Phylogeography of a species complex of lowland Neotropical rain forest trees (Carapa, Meliaceae)

AbstractAim Many tropical tree species have poorly delimited taxonomic boundaries and contain undescribed or cryptic species. We examined the genetic structure of a species complex in the tree genus Carapa in the Neotropics in order to evaluate age, geographic patterns of diversity and evolutionary relationships, and to quantify levels of introgression among currently recognized species.Location Lowland moist forests in the Guiana Shield, the Central and Western Amazon Basin, Chocó and Central America.Methods Genetic structure was analysed using seven nuclear simple sequence repeats (nuSSR), five chloroplast SSRs (cpSSR), and two chloroplast DNA (cpDNA) intergenic sequences (trnH–psbA and trnC–ycf6). Bayesian clustering analysis of the SSR data was used to infer population genetic structure and to assign 324 samples to their most likely genetic cluster. Bayesian coalescence analyses were performed on the two cpDNA markers to estimate evolutionary relationships and divergence times.Results Two genetic clusters (nu_guianensis and nu_surinamensis) were detected, which correspond to the Neotropical species C. guianensis (sensu latu) and C. surinamensis. Fourteen cpDNA haplotypes clustered into six haplogroups distributed between the two nuclear genetic clusters. Divergence between the haplogroups was initiated in the Miocene, with some haplotype structure evolving as recently as the Pleistocene. The absence of complete lineage sorting between the nuclear and chloroplast genomes and the presence of hybrid individuals suggest that interspecific reproductive barriers are incomplete. NuSSR diversity was highest in C. guianensis and, within C. guianensis, cpDNA diversity was highest in the Central and Western Amazon Basin. Regional genetic differentiation was strong but did not conform to an isolation‐by‐distance process or exhibit a phylogeographical signal.Main conclusions The biogeographical history of Neotropical Carapa appears to have been influenced by events that took place during the Neogene. Our results point to an Amazonian centre of origin and diversification of Neotropical Carapa, with subsequent migration to the Pacific coast of South America and Central America. Gene flow apparently occurs among species, and introgression events are supported by inconsistencies between chloroplast and nuclear lineage sorting. The absence of phylogeographical structure may be a result of the ineffectiveness of geographical barriers among populations and of reproductive isolation mechanisms among incipient and cryptic species in this species complex.

Aquaporins in the wild: natural genetic diversity and selective pressure in the PIP gene family in five Neotropical tree species.

BackgroundTropical trees undergo severe stress through seasonal drought and flooding, and the ability of these species to respond may be a major factor in their survival in tropical ecosystems, particularly in relation to global climate change. Aquaporins are involved in the regulation of water flow and have been shown to be involved in drought response; they may therefore play a major adaptive role in these species. We describe genetic diversity in the PIP sub-family of the widespread gene family of Aquaporins in five Neotropical tree species covering four botanical families.ResultsPIP Aquaporin subfamily genes were isolated, and their DNA sequence polymorphisms characterised in natural populations. Sequence data were analysed with statistical tests of standard neutral equilibrium and demographic scenarios simulated to compare with the observed results. Chloroplast SSRs were also used to test demographic transitions. Most gene fragments are highly polymorphic and display signatures of balancing selection or bottlenecks; chloroplast SSR markers have significant statistics that do not conform to expectations for population bottlenecks. Although not incompatible with a purely demographic scenario, the combination of all tests tends to favour a selective interpretation of extant gene diversity.ConclusionsTropical tree PIP genes may generally undergo balancing selection, which may maintain high levels of genetic diversity at these loci. Genetic variation at PIP genes may represent a response to variable environmental conditions.

Gene pool variation and phylogenetic relationships of an indigenous northeast Italian grapevine collection revealed by nuclear and chloroplast SSRs

A germplasm safeguard programme was set up with 19 grapevine varieties considered as indigenous to northeastern Italy. To better estimate how genetic structure can be used to obtain a conservation perspective of local varieties, genetic variability was examined at 30 nuclear and 3 chloroplast polymorphic microsatellite loci in the native varieties plus 7 European cultivars taken as reference. The genetic profiles of all the cultivars were searched for possible parentage relationships and several suspected cases of the same variety having different names were investigated. The alleles shared at the loci suggest a parent-offspring relationship between Merlot and Cabernet Franc, 'Gruaja' and 'Negrara Veronese', and Marzemina Nera and Marzemina Bianca. Alleles at the 30 nuclear loci are consistent with Raboso Veronese being the progeny of Marzemina Bianca and Raboso Piave. Chloroplast-specific haplotypes were singled out for the first time in this indigenous germplasm and should be considered typical of the region. It is hypothesized that there are many specific haplotypes for the local varieties due to a past contribution of wild grapevine to the cultivated gene pool. The majority of investigated cultivars were demonstrated to constitute an independent source of genetic variation, and therefore a possible valuable resource of genetic traits for breeders.