Shift In Reproductive Strategy Research Articles

Population genomics unveils the century-old invasion of the Seagrass Halophila stipulacea in the Mediterranean Sea

The tropical seagrass Halophila stipulacea invaded the Eastern Mediterranean Sea in the late nineteenth century and progressively spread throughout the basin ever since. Its spread is expected to continue north-westward as the Mediterranean Sea becomes warmer, potentially changing the seagrass biogeography of the basin. Given the power of genomics to assess invasion dynamics in non-model species, we report the first ddRAD-seq study of H. stipulacea and small-scale population genomic analysis addressing its century-old Mediterranean invasion. Based on 868 SNPs and 35 genotyped native (Red Sea) and exotic (from Cyprus, Greece, and Italy) samples, results suggest that genetic structure was high, especially between major geographic discontinuities, and that exotic populations maintain comparably lower genetic diversity than native populations, despite 130 years of invasion. The evidence of high heterozygosity excess, coupled with previously reported male-dominated and rare flowering records in the exotic range, suggests that clonal propagation likely played a pivotal role in the successful colonization and spread of H. stipulacea in the Mediterranean. This shift in reproductive strategy, particularly evident in the Italian populations located closest to the western boundary and representing more recent establishments, underscores the importance of this cost-effective mode of reproduction, especially during the initial stages of invasion, raising questions about the species future expansion trajectory. Our findings serve as a catalyst for future research into the species’ invasion dynamics, including deciphering the intricate roles of acclimatization and rapid adaptation, important for a comprehensive assessment of invasion risks and improving management strategies aimed at conserving seagrass ecosystems.

Reproductive strategy of an invasive buzz-pollinated plant (Solanum rostratum)

Some plants that invade environments outside their native range of distribution often face reproductive challenges such as limited pollinators and mates. Thus, the capacity to reproduce autonomously in the absence of pollinators or mates can facilitate establishment and spread of invasive plants. Here, we study the capacity of Solanum rostratum to reproduce through autonomous self-pollination. Although this plant is self-compatible, it strongly depends on visitation of buzzing bees to reproduce in its native range. We compared floral morphology, pollen: ovule ratios, reproductive phenology, and capacity for autonomous self-pollination of two populations of native distribution (Mexico) and another two populations of the invasive range (USA). We found a reduction in anther-stigma distance, pollen: ovule ratio, and plant height in invasive populations, as well as a shift to increased resource allocation to floral vs. vegetative structures and faster flowering compared to native populations. Importantly, invasive populations show a marked increase in the capacity to produce fruits autonomously (without pollen vectors). Our results suggest that the northern invasion of S. rostratum could have been facilitated by a shift in reproductive strategy from outcrossing to autonomous self-pollination.

Drivers of Batrachochytrium dendrobatidis infection load, with evidence of infection tolerance in adult male toads (Bufo spinosus)

Chytridiomycosis is affecting hundreds of amphibian species worldwide, but while in tropical areas, adult individuals have been the focus of most investigations, the exact role played by infection intensity of breeding adults is not well understood in temperate areas. We conducted mark–recapture–capture surveys during spiny common toad breeding seasons from 2006 to 2018 at the site of the first recorded outbreak of chytridiomycosis in Europe, the Peñalara Massif (Sierra de Guadarrama National Park, central Spain), and collected infection samples and several variables related to the reproductive effort of male individuals. We used general linear mixed models to evaluate the contribution of study variables on the infection loads of adult male toads exhibited at their capturing date. We also analysed the differences on several male characteristics between the pond with the largest breeding population against the rest of the ponds. We found that the duration of time spent in the waterbody and the condition of the host predicted infection loads. Animals of good physical condition, that spent longer in water, have higher infection levels than individuals with the opposite set of traits. The pond supporting the largest breeding population housed smaller male toads and in poorer condition. Our results are consistent with a shift in reproductive strategy in response to infection and potentially a strategy of tolerance, rather than resistance to infection. These findings have applications for disease mitigation and theoretical implications related to the trade-offs made and the evolution of traits in response to the disease.

A burst of genomic innovation at the origin of placental mammals mediated embryo implantation

The origin of embryo implantation in mammals ~148 million years ago was a dramatic shift in reproductive strategy, yet the molecular changes that established mammal implantation are largely unknown. Although progesterone receptor signalling predates the origin of mammals and is highly conserved in, and critical for, successful mammal pregnancy, it alone cannot explain the origin and subsequent diversity of implantation strategies throughout the placental mammal radiation. MiRNAs are known to be flexible and dynamic regulators with a well-established role in the pathophysiology of mammal placenta. We propose that a dynamic core microRNA (miRNA) network originated early in placental mammal evolution, responds to conserved mammal pregnancy cues (e.g. progesterone), and facilitates species-specific responses. Here we identify 13 miRNA gene families that arose at the origin of placental mammals and were subsequently retained in all descendent lineages. The expression of these miRNAs in response to early pregnancy molecules is regulated in a species-specific manner in endometrial epithelia of species with extreme implantation strategies (i.e. bovine and human). Furthermore, this set of miRNAs preferentially target proteins under positive selective pressure on the ancestral eutherian lineage. Discovery of this core embryo implantation toolkit and specifically adapted proteins helps explain the origin and evolution of implantation in mammals.

Temperature-driven shifts in the reproductive strategies of Centropages abdominalis Sato in Xiangshan Bay, China

Centropages abdominalis Sato, a common cold-water copepod that is distributed widely in the coastal waters of the North Pacific, appears seasonally in winter and spring as the key zooplankton in the Xiangshan Bay, a typical subtropical semi-closed bay in China. Here, we hypothesised that C. abdominalis could produce diapause eggs as an over-summering strategy in late spring as a response to the increasing temperature and day length. We conducted 11 cruises and in situ reproduction experiments from April 2017 to June 2018 to reveal the variation in the reproductive strategies of C. abdominalis and the related environmental factors. The results showed that the revival of diapause eggs in the inner and middle part of the Xiangshan Bay should contribute to the initial population recruitment in early winter. Sustaining subitaneous egg production supported the instantaneous population recruitment in later winter and spring, and diapause eggs were mainly produced at the upper thermal distribution of C. abdominalis (16.12–21.32 °C) to over-summer in late spring. Temperature was the essential environmental factor that affected the shift in reproductive strategy for C. abdominalis, with an optimum temperature of 19.51 °C for diapause egg production and 11.63 °C for subitaneous one in the Xiangshan Bay. Furthermore, the long-day photoperiod possibly acted as a seasonal change signal for C. abdominalis to produce diapause eggs in late spring in the Xiangshan Bay. The timing of diapause egg production for C. abdominalis could be earlier in lower-latitude waters with shorter day length, as an acclimatisation to warmer habitats. Besides, salinity and Chl a concentration had less effects on diapause eggs production of C. abdominalis in this case. Confronted with ocean warming, the diapause egg period of cold-water copepod (like C. abdominalis) would be prolongated, leading to a reduced population appearance period, especially in lower-latitude waters.

Host-Parasite Co-Evolution in Real-Time: Changes in Honey Bee Resistance Mechanisms and Mite Reproductive Strategies.

Simple SummaryParasitic mites, Varroa destructor, are a major threat for Western honey bees, Apis mellifera, colonies globally. Nevertheless, some honey bee populations can survive infestations with this mite, probably due to behaviors that suppress parasite reproduction. However, possible changes in mites associated with these surviving bees and the potential variations of bee behavior over time are poorly understood. Here, we show that mites can change their reproduction when associated with surviving hosts and that the bee behaviors suppressing mite reproduction can vary over time. In a fully-crossed field experiment on Dutch surviving colonies (Amsterdam Water Dunes (AWD) selection), worker brood cell recapping and varroa sensitive hygiene (VSH) performed by bees and mite reproductive parameters were investigated. Neither recapping nor VSH were significantly expressed even though a previous study showed VSH in these AWD bees. A larger proportion of mites that co-evolved with AWD surviving bees reproduced compared to mites in routinely treated colonies, but had lower fecundity. Overall, our study suggests that honeybee colonies can survive infestations with these mites by not yet understood means and shows for the first time adaptive changes in the reproduction of their coevolved mites.Co-evolution is a major driving force shaping the outcome of host-parasite interactions over time. After host shifts, the lack of co-evolution can have a drastic impact on novel host populations. Nevertheless, it is known that Western honey bee (Apis mellifera) populations can cope with host-shifted ectoparasitic mites (Varroa destructor) by means of natural selection. However, adaptive phenotypic traits of the parasites and temporal variations in host resistance behavior are poorly understood. Here, we show that mites made adaptive shifts in reproductive strategy when associated with resistant hosts and that host resistance traits can change over time. In a fully-crossed field experiment, worker brood cells of local adapted and non-adapted (control) A. mellifera host colonies were infested with mites originating from both types of host colonies. Then, mite reproduction as well as recapping of cells and removal of infested brood (i.e., Varroa Sensitive Hygiene, VSH) by host workers were investigated and compared to data from the same groups of host colonies three years earlier. The data suggest adaptive shifts in mite reproductive strategies, because mites from adapted hosts have higher probabilities of reproduction, but lower fecundity, when infesting their associated hosts than mites in treated colonies. The results confirm that adapted hosts can reduce mite reproductive success. However, neither recapping of cells nor VSH were significantly expressed, even though the latter was significantly expressed in this adapted population three years earlier. This suggests temporal variation in the expression of adaptive host traits. It also appears as if mechanisms not investigated here were responsible for the reduced mite reproduction in the adapted hosts. In conclusion, a holistic view including mite adaptations and studies of the same parasite/host populations over time appears overdue to finally understand the mechanisms enabling survival of V. destructor-infested honey bee host colonies.

Long-term survey of sea turtles (Caretta caretta) reveals correlations between parasite infection, feeding ecology, reproductive success and population dynamics

Long-term monitoring of host-parasite interactions is important for understanding the consequences of infection on host fitness and population dynamics. In an eight-year survey of the loggerhead sea turtle (Caretta caretta) population nesting in Cabo Verde, we determined the spatiotemporal variation of Ozobranchus margoi, a sanguivorous leech best known as a vector for sea turtle fibropapilloma virus. We quantified O. margoi association with turtles’ δ15N and δ13C stable isotopes to identify where infection occurs. We then measured the influence of infection on reproduction and offspring fitness. We found that parasite prevalence has increased from 10% of the population in 2010, to 33% in 2017. Stable isotope analysis of host skin samples suggests transmission occurs within the host’s feeding grounds. Interestingly, we found a significant interaction between individual size and infection on the reproductive success of turtles. Specifically, small, infected females produced fewer offspring of poorer condition, while in contrast, large, infected turtles produced greater clutch sizes and larger offspring. We interpret this interaction as evidence, upon infection, for a size-dependent shift in reproductive strategy from bet hedging to terminal investment, altering population dynamics. This link between infection and reproduction underscores the importance of using long-term monitoring to quantify the impact of disease dynamics over time.

The homothallic mating-type locus of the conifer needle endophyte Phialocephala scopiformis DAOMC 229536 (order Helotiales)

We describe the complete mating-type (MAT) locus for Phialocephala scopiformis Canadian Collection of Fungal Cultures (DAOMC) 229536 – a basal lineage within Vibrisseaceae. This strain is of interest due to its ability to produce the important antiinsectan rugulosin. We also provide some of the first insights into the genome structure and gene inventory of nonclavicipitalean endophytes. Sequence was obtained through shotgun sequencing of the entire P. scopiformis genome, and the MAT locus was then determined by comparing this genomic sequence to known MAT loci within the Phialocephala fortinii s.l.–Acephala applanata species complex. We also tested the relative levels of sequence conservation for MAT genes within Vibrisseaceae (n = 10), as well as within the Helotiales (n = 27). Our results: (1) show a homothallic gene arrangement for P. scopiformis [MAT1-1-1, MAT1-2-1, and MAT1-1-3 genes are present], (2) increase the genomic survey of homothallism within Vibrisseaceae, (3) confirm that P. scopiformis contains a unique S-adenosyl-l-methionine-dependent methyltransferase (SAM-Mtase) gene proximal to its MAT locus, while also lacking a cytoskeleton assembly control protein (sla2) gene, and (4) indicate that MAT1-1-1 is the more useful molecular marker amongst the MAT genes for phylogenetic reconstructions aimed at tracking evolutionary shifts in reproductive strategy and/or MAT loci gene composition within the Helotiales.

From endogenous to exogenous pattern formation: Invasive plant species changes the spatial distribution of a native ant.

Invasive species are a significant threat to global biodiversity, but our understanding of how invasive species impact native communities across space and time remains limited. Based on observations in an old field in Southeast Michigan spanning 35years, our study documents significant impacts of habitat change, likely driven by the invasion of the shrub, Elaeagnus umbellata, on the nest distribution patterns and population demographics of a native ant species, Formica obscuripes. Landcover change in aerial photographs indicates that E.umbellata expanded aggressively, transforming a large proportion of the original open field into dense shrubland. By comparing the ant's landcover preferences before and after the invasion, we demonstrate that this species experienced a significant unfavorable change in its foraging areas. We also find that shrub landcover significantly moderates aggression between nests, suggesting nests are more related where there is more E.umbellata. This may represent a shift in reproductive strategy from queen flights, reported in the past, to asexual nest budding. Our results suggest that E.umbellata may affect the spatial distribution of F.obscuripes by shifting the drivers of nest pattern formation from an endogenous process (queen flights), which led to a uniform pattern, to a process that is both endogenous (nest budding) and exogenous (loss of preferred habitat), resulting in a significantly different clustered pattern. The number and sizes of F.obscuripes nests in our study site are projected to decrease in the next 40years, although further study of this population's colony structures is needed to understand the extent of this decrease. Elaeagnus umbellata is a common invasive shrub, and similar impacts on native species might occur in its invasive range, or in areas with similar shrub invasions.

Evidence of positive selection associated with placental loss in tiger sharks.

BackgroundAll vertebrates initially feed their offspring using yolk reserves. In some live-bearing species these yolk reserves may be supplemented with extra nutrition via a placenta. Sharks belonging to the Carcharhinidae family are all live-bearing, and with the exception of the tiger shark (Galeocerdo cuvier), develop placental connections after exhausting yolk reserves. Phylogenetic relationships suggest the lack of placenta in tiger sharks is due to secondary loss. This represents a dramatic shift in reproductive strategy, and is likely to have left a molecular footprint of positive selection within the genome.ResultsWe sequenced the transcriptome of the tiger shark and eight other live-bearing shark species. From this data we constructed a time-calibrated phylogenetic tree estimating the tiger shark lineage diverged from the placental carcharhinids approximately 94 million years ago. Along the tiger shark lineage, we identified five genes exhibiting a signature of positive selection. Four of these genes have functions likely associated with brain development (YWHAE and ARL6IP5) and sexual reproduction (VAMP4 and TCTEX1D2).ConclusionsOur results indicate the loss of placenta in tiger sharks may be associated with subsequent adaptive changes in brain development and sperm production.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0696-y) contains supplementary material, which is available to authorized users.

Fire Elicits an Adaptive Reproductive Strategy Shift from Host Plant Quantity to Quality in a Capital Breeding Species

Capital breeding species, those that do not acquire resources over their reproductive period, are hypothesized to have more flexible reproductive strategies than income breeding species, enabling the former to better cope with environmental changes. Yet, empirical study of this life history attribute in a changing environment is rare. Hemileuca eglanterina (Saturniidae), a strict capital breeding, day-flying moth, should employ a different reproductive strategy to exploit temporary increases in host plant nutrient quality following fire. In wetlands, where one half was burned and the other left unburned, the number of eggs/clutch was positively correlated with host plant abundance in the absence of fire, suggesting that H. eglanterina uses a resource abundance matching reproductive strategy by default. However, following fire, H. eglanterina laid greater numbers of eggs/clutch and did not adjust clutch size to host plant abundance, appearing to shift to a host plant quality based reproductive strategy. Coinciding with the fire-induced shift in reproductive strategy, host plants from burned habitat contained a greater proportion of N-containing compounds in their foliage than plants from unburned habitat, and larvae fed only leaves from the burned habitat had significantly greater survival than siblings fed unburned foliage. These results suggest the shift in reproductive strategy by H. eglanterina following fire was adaptive and that capital breeding species can cope with sudden environmental changes via alternative reproductive strategies.

The timing of molecular and morphological changes underlying reproductive transitions in wild tomatoes (Solanumsect.Lycopersicon)

Molecular mechanisms underlying the transition from genetic self-incompatibility to self-compatibility are well documented, but the evolution of other reproductive trait changes that accompany shifts in reproductive strategy (mating system) remains comparatively under-investigated. A notable exception is the transition from exserted styles to styles with recessed positions relative to the anthers in wild tomatoes (Solanum Section Lycopersicon). This phenotypic change has been previously attributed to a specific mutation in the promoter of a gene that influences style length (style2.1); however, whether this specific regulatory mutation arose concurrently with the transition from long to short styles, and whether it is causally responsible for this phenotypic transition, has been poorly investigated across this group. To address this gap, we assessed 74 accessions (populations) from 13 species for quantitative genetic variation in floral and reproductive traits as well as the presence/absence of deletions at two different locations (StyleD1 and StyleD2) within the regulatory region upstream of style2.1. We confirmed that the putatively causal deletion variant (a 450-bp deletion at StyleD1) arose within self-compatible lineages. However, the variation and history of both StyleD1 and StyleD2 was more complex than previously inferred. In particular, although StyleD1 was statistically associated with differences in style length and stigma exsertion across all species, we found no evidence for this association within two species polymorphic for the StyleD1 mutation. We conclude that the previous association detected between phenotypic and molecular differences is most likely due to a phylogenetic association rather than a causal mechanistic relationship. Phenotypic variation in style length must therefore be due to other unexamined linked variants in the style2.1 regulatory region.

Morphological changes and resource allocation of Zizania latifolia (Griseb.) Stapf in response to different submergence depth and duration

Plants around ponds, rivers and lakes are subjected to long-term partial or complete submergence. When they are flooded, water level affects the plants simultaneously with duration of submergence. Separate and interactive effects of water level and duration on the growth of the herbaceous perennial Zizania latifolia (Poaceae) were investigated by exposing the plants in greenhouse water tanks to submergence in different water depths and for different time-spans. The plants exhibited great shoot elongation upon submergence and prolonged flood duration, and the basal tiller number of the species decreased with higher water levels. Submergence treatment advanced the flowering date and increased the inflorescence number. Plant total biomass did not differ among all the treatments, while the root:shoot ratio decreased with increased water level, prolonged duration of submergence and their interaction. The high plasticity in morphology and shifts in reproductive strategy and biomass allocation enabled the Zizania plants to survive the compound effect of flooding height and duration. This may explain the occurrence of this species in habitats subjected to long-term flooding. The results obtained in this experiment will contribute to understanding the impact of flooding dynamics on plants and the ways of adaptation responses to prolonged waterlogging.

Mixed asexual and sexual reproduction in the Indo-Pacific reef coral Pocillopora damicornis.

Pocillopora damicornis is one of the best studied reef-building corals, yet it's somewhat unique reproductive strategy remains poorly understood. Genetic studies indicate that P. damicornis larvae are produced almost exclusively parthenogenetically, and yet population genetic surveys suggest frequent sexual reproduction. Using microsatellite data from over 580 larvae from 13 colonies, we demonstrate that P. damicornis displays a mixed reproductive strategy where sexual and asexual larvae are produced simultaneously within the same colony. The majority of larvae were parthenogenetic (94%), but most colonies (10 of the 13) produced a subset of their larvae sexually. Logistic regression indicates that the proportion of sexual larvae varied significantly with colony size, cycle day, and calendar day. In particular, the decrease in sexual larvae with colony size suggests that the mixed reproductive strategy changes across the life of the coral. This unique shift in reproductive strategy leads to increasingly asexual replications of successful genotypes, which (in contrast to exclusive parthenogens) have already contributed to the recombinant gene pool.

Risk of predation reflects variation in the reproductive strategy of a dominant forage fish in mangrove tidal tributaries

The role of predators in shaping prey life histories is a central theme in the ecological literature. However, the association between degree of predation risk and prey reproductive strategies has not been clearly established. We examined reproduction in the sailfin molly (Poecilia latipinna) from small tidal tributaries in a subtropical estuary. Our results revealed a gradient along which females produced many, small offspring at one extreme (mean = 42 offspring, 17 mg each) and fewer, larger offspring at the other (24 offspring, 29 mg each). Reproductive allotment ranged from 14.9-21.5% of maternal biomass. Based on our observation of divergent reproductive strategies, we experimentally tested the null hypothesis of no difference in predation risk among tributaries using a novel quantitative approach to estimate predation. We predicted greater risk in tributaries where mollies produced many, small offspring. Tethering confirmed increasing risk from 16.2 ± 5.3% SE to 54.7 ± 3.6% fish lost h(-1) across sites in agreement with observed variation in reproduction. Predation was unexpectedly higher than predicted at one of the four sites suggesting that additional factors (e.g., food) had influenced reproduction there. Our results provide insight into the well-studied concept of predator-mediated variation in prey reproduction by quantitatively demonstrating differential risk for mollies exhibiting divergent reproductive strategies. While the observed range of variation in reproductive traits was consistent with previous studies reporting strong predator effects, higher than expected predation in one case may suggest that the prey response does not follow a continuous trajectory of incremental change with increasing predation risk, but may be better defined as a threshold beyond which a significant shift in reproductive strategy occurs.

Sexual partners for the stressed: facultative outcrossing in the self-fertilizing nematode Caenorhabditis elegans.

Sexual reproduction shuffles genetic variation, potentially enhancing the evolutionary response to environmental change. Many asexual organisms respond to stress by generating facultative sexual reproduction, presumably as a means of escaping the trap of low genetic diversity. Self-fertilizing organisms are subject to similar genetic limitations: the consistent loss of genetic diversity within lineages restricts the production of variation through recombination. Selfing organisms may therefore benefit from a similar shift in mating strategy during periods of stress. We determined the effects of environmental stress via starvation and passage through the stress-resistant dauer stage on mating system dynamics of Caenorhabditis elegans, which reproduces predominantly through self-fertilization but is capable of outcrossing in the presence of males. Starvation elevated male frequencies in a strain-specific manner through differential male survival during dauer exposure and increased outcrossing rates after dauer exposure. In the most responsive strain, the mating system changed from predominantly selfing to almost exclusively outcrossing. Like facultative sex in asexual organisms, facultative outcrossing in C. elegans may periodically facilitate adaptation under stress. Such a shift in reproductive strategy should have a major impact on evolutionary change within these populations and may be a previously unrecognized feature of other highly selfing organisms.

Winged queens replaced by reproductives smaller than workers in Mystrium ants

In ants, winged queens that are specialized for independent colony foundation can be replaced by wingless reproductives better adapted for colony fission. We studied this shift in reproductive strategy by comparing two Mystrium species from Madagascar using morphometry, allometry and dissections. Mystrium rogeri has a single dealate queen in each colony with a larger thorax than workers and similar mandibles that allow these queens to hunt during non-claustral foundation. In contrast, Mystrium 'red' lacks winged queens and half of the female adults belong to a wingless 'intermorph' caste smaller and allometrically distinct from the workers. Intermorphs have functional ovaries and spermatheca while those of workers are degenerate. Intermorphs care for brood and a few mate and reproduce making them an all-purpose caste that takes charge of both work and reproduction. However, their mandibles are reduced and inappropriate for hunting centipedes, unlike the workers' mandibles. This together with their small thorax disallow them to perform independent colony foundation, and colonies reproduce by fission. M. rogeri workers have mandibles polymorphic in size and shape, which allow for all tasks from brood care to hunting. In M. 'red', colonial investment in reproduction has shifted from producing expensive winged queens to more numerous helpers. M. 'red' intermorphs are the first case of reproductives smaller than workers in ants and illustrate their potential to diversify their caste system for better colonial economy.

ORIGIN AND RAPID DIVERSIFICATION OF A TROPICAL MOSS

Molecular sequences rarely evolve at a constant rate. Yet, even in instances where a clock can be assumed or approximated for a particular set of sequences, fossils or clear patterns of vicariance are rarely available to calibrate the clock. Thus, obtaining absolute timing for diversification of natural lineages can prove difficult. Unfortunately, without absolute time we cannot develop a complete understanding of important evolutionary processes, including adaptive radiations and key innovations. In the present study, the coding sequence of the nuclear gene, glyceraldehyde 3-phosphate dehydrogenase (gpd), extracted from the paleotropical moss, Mitthyridium, was found to exhibit clocklike behavior and used to reconstruct the history of 80 distinct molecular lineages that cover the full geographic range of Mitthyridium. Two separate clades endemic to two geographically distinct oceanic archipelagos were revealed by this phylogenetic analysis. This allowed the use of island age (as derived from potassium-argon dating) as a maximum age of origin of each monophyletic group, providing two independent time anchors for the clock found in gpd, the final piece needed to study absolute time. Based on results from both maximum age calibrations, which separately yielded highly consistent estimates, the ancestor of this moss group arose approximately 8 million years ago, and then diversified at the rapid rate of 0.56 +/- 0.004 new lineages per million years. Such a rate is on par with the highest diversification rates reported in the literature including rapidly radiating insular groups like the Hawaiian silversword alliance, a classic example of an adaptive radiation. Using independent sources of data, it was found that neither the age nor diversification estimates were affected by the use of molecular lineages rather than species as the operational taxonomic units. Identifying the cause for this rapid diversification requires further testing, but it appears to be related to a general shift in reproductive strategy from sexual to asexual, which may be a key innovation for this young group.

ORIGIN AND RAPID DIVERSIFICATION OF A TROPICAL MOSS

Molecular sequences rarely evolve at a constant rate. Yet, even in instances where a clock can be assumed or approximated for a particular set of sequences, fossils or clear patterns of vicariance are rarely available to calibrate the clock. Thus, obtaining absolute timing for diversification of natural lineages can prove difficult. Unfortunately, without absolute time we cannot develop a complete understanding of important evolutionary processes, including adaptive radiations and key innovations. In the present study, the coding sequence of the nuclear gene, glyceraldehyde 3-phosphate dehydrogenase (gpd), extracted from the paleotropical moss, Mitthyridium, was found to exhibit clocklike behavior and used to reconstruct the history of 80 distinct molecular lineages that cover the full geographic range of Mitthyridium. Two separate clades endemic to two geographically distinct oceanic archipelagos were revealed by this phylogenetic analysis. This allowed the use of island age (as derived from potassium-argon dating) as a maximum age of origin of each monophyletic group, providing two independent time anchors for the clock found in gpd, the final piece needed to study absolute time. Based on results from both maximum age calibrations, which separately yielded highly consistent estimates, the ancestor of this moss group arose approximately 8 million years ago, and then diversified at the rapid rate of 0.56 ± 0.004 new lineages per million years. Such a rate is on par with the highest diversification rates reported in the literature including rapidly radiating insular groups like the Hawaiian silversword alliance, a classic example of an adaptive radiation. Using independent sources of data, it was found that neither the age nor diversification estimates were affected by the use of molecular lineages rather than species as the operational taxonomic units. Identifying the cause for this rapid diversification requires further testing, but it appears to be related to a general shift in reproductive strategy from sexual to asexual, which may be a key innovation for this young group.

Liriodendroid seeds from the Late Cretaceous of Kazakhstan and North Carolina, USA

Three new species ( Liriodendroidea alata, L. latirapha, and L. carolinensis) are established based on well-preserved seeds from the Late Cretaceous of Kazakhstan and eastern North America. The Kazakhstan material is Cenomanian-Turonian in age and represents the oldest well-supported occurrence of the Magnoliaceae in the Cretaceous. The North American material is slightly younger (Campanian). The seeds described here share several critical characters with seeds of extant and Tertiary species of Liriodendron (Magnoliaceae) including anatropous organization and endotestal seed coat structure with a distinct endotesta of palisade-shaped sclerenchyma cells containing fibrous lignifications and cubic crystals. Other characters shared with Liriodendron and other Magnoliaceae include the characteristic fingerprint-like pattern on the surface of the endotesta and formed from the cells of the mesotesta, the presence of a distinct heteropyle, and a thin, crushed tegmen. The Cretaceous seeds from Kazakhstan and North Carolina differ, however, from those of modern Liriodendron in their smaller size and in having a distinct wing indicating that seeds were dispersed separately by wind in contrast to seeds of Liriodendron that are dispersed within the indehiscent and winged fruitlets. The occurrence of several adhering seeds documents that the fruitlets in at least two species of Liriodendroidea contained more than two seeds, a feature which indicates a shift in reproductive strategy in the tribe Liriodendreae from the Cretaceous to the present.