Climate,Colonisation and Celibacy: Population Structure in Central European Trichomanes speciosum (Pteridophyta)

The Killarney fern Trichomanes speciosum Willd. (Hymenophyllaceae) is unique in possessing both extensive sexual (sporophyte and gametophyte generations present) and asexual (gametophyte only) ranges. It was first discovered in central Europe in 1993 and is represented in this area only by its perennial, vegetatively propagating gametophyte generation. Genetic variation has been investigated at 35 sites. Allozyme diversity is partitioned primarily between, not within, sites. Although genetic variation exists at a fine scale (lt 5 m) within some populations, the results suggest that clones were not intimately associated in these cases. The majority of sites support unique multilocus phenotypes. Where phenotypes were present at more than one site they tended to recur at the next closest site. However, similar phenotypes link eastern and western Pfälzerwald sites up to c. 70 km apart. This pattern of diversity suggests that colonisation was not solely of a “stepping stone” or “leading edge” type. We suggest that during a climatically favourable period, probably the Atlantic hypsithermal, there may have been an explosive colonisation by long-distance dispersal from refugial areas. This was followed by a short period during which sporophyte production, sexual reproduction and local spread were possible. With climatic change, reduction in the available habitat and the loss of the sporophyte generation, different individual genets became fixed within small, favourable, but scattered, sites. The possibility that some central European sites north of the Alps acted as periglacial refugia cannot be discounted, but would appear less likely than (re-) colonisation from the Atlantic fringe.     

Patterns of Clonal Diversity in Dicranopteris linearis on Mauna Loa,Hawaii1

The clonal mat-forming fern, Dicranopteris linearis (N. L. Burm.) Underw., dominates vast areas of rainforests on the windward slopes of Mauna Loa Volcano on the island of Hawai'i. Because clone size has important ecological and evolutionary consequences in such a dominant species, we used isozyme analysis to investigate clone size and other aspects of genetic diversity and reproduction over a broad range of environmental conditions on primary successional sites (pahoehoe lava substrates). Isozyme analysis provided a measure of the upper limit of clonal size in this interdigitating clonal species. Each 0.5-ha primary successional site on Mauna Loa was comprised of a minimum of two to four clones. Genetic diversity in Dicranopteris was low; of 32 putative loci investigated, only 4 were polymorphic, with 2 or 3 alleles/locus. Over the 17 study locations on Mauna Loa and Kilauea Volcanoes, we identified nine multilocus genotypes based on unique combinations of allozymes. Seven of the nine genotypes were heterozygous for at least one locus, evidence of an intergametophytic mating system. Highly dispersible spores, coupled with intergametophytic mating should promote higher genetic diversity. We propose that the following factors contributed to low genetic diversity: founder effects; extreme isolation from mainland gene pools; high potential for mating among different gametophytes produced from the same sporophyte; relatively low numbers of safe sites for gametophyte establishment over space and time; and long-term reliance on vegetative growth. Leaf phenotypes were associated with genotype, but also with environmental conditions. Enough variability within a genotype existed to support the current treatment of Hawaiian Dicranopteris as one species. Vegetative growth was the primary means by which Dicranopteris covered the landscape. Nevertheless, spore production, gametophyte establishment, and sexual reproduction were absolutely essential for colonization of the few favorable microsites available on pahoehoe lava substrates of Mauna Loa following lava eruptions, dieback, and similar landscape-level disturbances.     

苔藓植物生殖生态学研究

近年来苔藓植物生殖生态学研究主要集中于繁育系统、生殖代价与对策,以及不同生殖方式对种群遗传变异的影响等方面。生殖结构的原始性及其对水分的独特需求,以及雌雄异株比例较高等导致苔藓植物中有性生殖比例偏低;雌配子体很少完成整个有性生殖过程,其“真实的生殖代价”主要指雌性性表达(雌配子发生)的能耗,并且显著低于雄性性表达;基于对资源有效分配的生殖对策而导致雌性偏向及部分孢子体败育。无性生殖有利于不同生境条件下有效种群的发展与维持,其多样化的繁殖方式导致复杂的种群动态。苔藓植物具有较高的种群遗传多样性,生殖方式与种群遗传变异无直接因果关系,孢子与无性繁殖体不同的散布能力对于种群间遗传分化具有一定的影响。     

雄配子体选择的遗传分化效应及其在植物改良中的应用

在植物世代交替的生活史中,配子体是产生配子和具有单倍数染色体的植物体,并且有自己的遗传表达信息.在配子产生的过程中以及配子结合之前,适者生存的法则对配子发挥着选择作用,只有最适应外界环境条件的配子才能通过竞争性受精并产生合子.雄配子体选择是影响植物遗传分化、演变和遗传多样性的重要因素,被认为是生物进化的有力动力.此外,由于植物基因组中约有2/3的基因表达交错发生在配子体阶段和孢子体阶段,因此雄配子体选择的结果会影响到下一代孢子体的表现型.在育种实践中,利用雄配子体选择对植物进行遗传改良,具有提高选择机率和缩短育种年限等优点.本文主要概述雄配子体选择与孢子体表型的关刎系、遗传分化效应及其在植物遗传改良中的应用,以期为雄配子选择相关研究提供参考     

Genetic Evidence of the Contribution of Ethnic Migrations to the Propagation and Persistence of the Rare and Declining Scrambling Shrub Caesalpinia bonduc L

This paper examines the contribution of human migrations to the propagation and maintenance of Caesalpina bonduc by means of an analysis of its population genetics and distribution patterns. One hundred and forty seven sites were surveyed in the three climatic zones of Benin and all individuals of the species were recorded. A set of individuals was randomly selected and sampled from seven populations and morphological variation and genetic diversity were assessed. The study confirmed the presence of the species in all climatic zones but its abundance varied greatly. Morphological variability between populations and zones was low in comparison with the high amount of variation within populations. AFLP and cpDNA fingerprinting revealed an extremely low genetic diversity within populations and a low genetic differentiation, suggesting parental links between populations. The results support the hypothesis of human involvement in Caesalpinia dispersal and persistence in Benin. However, the low genetic diversity may imply high risks for future extinction. We recommend that gene flow among the remaining populations be supported in order to conserve the species.     

Reproductive strategy, clonal structure and genetic diversity in populations of the aquatic macrophyte Sparganium emersum in river systems

Many aquatic and riparian plant species are characterized by the ability to reproduce both sexually and asexually. Yet, little is known about how spatial variation in sexual and asexual reproduction affects the genotypic diversity within populations of aquatic and riparian plants. We used six polymorphic microsatellites to examine the genetic diversity within and differentiation among 17 populations (606 individuals) of Sparganium emersum, in two Dutch-German rivers. Our study revealed a striking difference between rivers in the mode of reproduction (sexual vs. asexual) within S. emersum populations. The mode of reproduction was strongly related to locally reigning hydrodynamic conditions. Sexually reproducing populations exhibited a greater number of multilocus genotypes compared to asexual populations. The regional population structure suggested higher levels of gene flow among sexually reproducing populations compared to clonal populations. Gene flow was mainly mediated via hydrochoric dispersal of generative propagules (seeds), impeding genetic differentiation among populations even over river distances up to 50 km. Although evidence for hydrochoric dispersal of vegetative propagules (clonal plant fragments) was found, this mechanism appeared to be relatively less important. Bayesian-based assignment procedures revealed a number of immigrants, originating from outside our study area, suggesting intercatchment plant dispersal, possibly the result of waterfowl-mediated seed dispersal. This study demonstrates how variation in local environmental conditions in river systems, resulting in shifting balances of sexual vs. asexual reproduction within populations, will affect the genotypic diversity within populations. This study furthermore cautions against generalizations about dispersal of riparian plant species in river systems.     

Adaptations to extreme low light in the fern Trichomanes speciosum

Trichomanes speciosum is a threatened species restricted to sheltered, very humid sites. Uniquely amongst European ferns, differing ecological tolerances of the gametophyte and sporophyte generations are manifested as widely differing distributions. The perennial, vegetatively propagating gametophyte persists in drier, colder, darker habitats than the sporophyte. In sites where the gametophyte grows, light availability was found to be < 1 μmol m² s¹ for approx. 85% of daylight hours, rarely or (in some sites) never rising above 10 μmol m² s¹. Much of the time, light was < 0.01% of full sunlight. Measurements of gas exchange and chlorophyll fluorescence yield show that these plants have optimal photosynthesis at light intensities c . 5–10 μmol m² s¹, the highest light to which they are normally exposed to in their natural environment. The absence of any capacity for reversible nonphotochemical fluorescence quenching means that there is little or no protection of the photosynthetic apparatus from light-induced damage. We conclude that these plants are able to create what are essentially monocultures in their extreme environments only because of a combination of low metabolic rate (at low temperatures) and an ability to make efficient use of what little light is available to them by morphological and physiological means.     

Climatic changes can drive the loss of genetic diversity in a Neotropical savanna tree species

The high rates of future climatic changes, compared with the rates reported for past changes, may hamper species adaptation to new climates or the tracking of suitable conditions, resulting in significant loss of genetic diversity. Trees are dominant species in many biomes and because they are long‐lived, they may not be able to cope with ongoing climatic changes. Here, we coupled ecological niche modelling (ENM) and genetic simulations to forecast the effects of climatic changes on the genetic diversity and the structure of genetic clusters. Genetic simulations were conditioned to climatic variables and restricted to plant dispersal and establishment. We used a Neotropical savanna tree as species model that shows a preference for hot and drier climates, but with low temperature seasonality. The ENM predicts a decreasing range size along the more severe future climatic scenario. Additionally, genetic diversity and allelic richness also decrease with range retraction and climatic genetic clusters are lost for both future scenarios, which will lead genetic variability to homogenize throughout the landscape. Besides, climatic genetic clusters will spatially reconfigure on the landscape following displacements of climatic conditions. Our findings indicate that climate change effects will challenge population adaptation to new environmental conditions because of the displacement of genetic ancestry clusters from their optimal conditions.     

Sexual conflict and the alternation of haploid and diploid generations

Land plants possess a multicellular diploid stage (sporophyte) that begins development while attached to a multicellular haploid progenitor (gametophyte). Although the closest algal relatives of land plants lack a multicellular sporophyte, they do produce a zygote that grows while attached to the maternal gametophyte. The diploid offspring shares one haploid set of genes with the haploid mother that supplies it with resources and a paternal haploid complement that is not shared with the mother. Sexual conflict can arise within the diploid offspring because the offspring's maternal genome will be transmitted in its entirety to all other sexual and asexual offspring that the mother may produce, but the offspring's paternally derived genes may be absent from these other offspring. Thus, the selective forces favouring the evolution of genomic imprinting may have been present from the origin of modern land plants. In bryophytes, where gametophytes are long-lived and capable of multiple bouts of asexual and sexual reproduction, we predict strong sexual conflict over allocation to sporophytes. Female gametophytes of pteridophytes produce a single sporophyte and often lack means of asexual reproduction. Therefore, sexual conflict is predicted to be attenuated. Finally, we explore similarities among models of mate choice, offspring choice and segregation distortion.     

Population structure is not a simple function of reproductive mode and larval type: insights from tropical corals

1. For a wide range of organisms, heritable variation in life-history characteristics has been shown to be strongly subject to selection, reflecting the impact that variation in characters such as genotypic diversity, duration of larval development and adaptations for dispersal can have on the fitness of offspring and the make-up of populations. Indeed, variation in life-history characteristics, especially reproduction and larval type, have often been used to predict patterns of dispersal and resultant population structures in marine invertebrates. 2. Scleractinian corals are excellent models with which to test this relationship, as they exhibit almost every possible combination of reproductive mode and larval type. Some general patterns are emerging but, contrary to expectations, genetic data suggest that while populations of broadcast spawning species may be genotypically diverse they may be heavily reliant on localized recruitment rather than widespread dispersal of larvae. 3. Here we use microsatellites to test the importance of localized recruitment by comparing the genetic structure of populations of two broadcast spawning corals with contrasting modes of reproduction and larval development; Goniastrea favulus is self-compatible, has sticky, negatively buoyant eggs and larvae and is expected to have restricted dispersal of gametes and larvae. In contrast, Platygyra daedalea is self-incompatibile, spawns positively buoyant egg-sperm bundles and has planktonic development. 4. Surprisingly, spatial-autocorrelation revealed no fine-scale clustering of similar genotypes within sites for G. favulus, but showed a non-random distribution of genotypes in P. daedalea. Both species showed similar levels of genetic subdivision among sites separated by 50-100 m (F(ST) = 0.03), suggesting that larval dispersal may be equivalent in both species. 5. Interestingly, as fragmentation has been considered rare in massive corals, our sample of 284 P. daedalea colonies included 28 replicated genotypes that were each unlikely (P < 0.05) to have been derived independently from sexual reproduction. 6. We conclude that the extreme life history of G. favulus does not produce unusually fine-scale genetic structure and subsequently, that reproductive mode and larval type may not be not good predictors of population structure or dispersal ability.     

Dispersal and genetic differentiation of Syntrichia caninervis populations across different desert regions in China

• The moss Syntrichia caninervis is widely distributed in cool temperate and cold deserts where environmental pressures create a dependence on asexual reproduction (fragment reproduction). However, when compared to sporophyte‐producing mosses, there is a lack of evidence to support the capacity of drought‐tolerant mosses that predominantly fragment and produce protonema to disperse over long distances.
• We used 20 microsatellite loci to study genetic variation and structure in six populations (five natural populations and one population from a regeneration site) in three contrasting and widely separated regions of China.
• The genetic diversity and expected heterozygosity were lower in populations from the Tengger Desert than in populations from the other regions. Using PCoA, UPGMA and Structure analysis, the genetic grouping divided the three regions into three distinct groups. This may indicate that in regions where S. caninervis reproduces predominantly asexually, propagules are spread mainly by short‐distance dispersal. The genetic diversity of the population from the regeneration site in the Tengger Desert was slightly higher than that of the nearby, naturally occurring population, and included some input from the Pamir Plateau almost 2,300 km to the west, suggesting long‐distance dispersal of S. caninervis propagules across the region.
• Predominantly asexually reproducing populations of S. caninervis are mainly dependent on short‐distance dispersal. Long‐distance dispersal of S. caninervis propagules across the region is difficult. Establishment of populations with dominant asexual reproduction will eventually result in genetic differentiation.

     

Variation within and between Closely Related Species Uncovers High Intra-Specific Variability in Dispersal

Mounting evidence shows that contrasting selection pressures generate variability in dispersal patterns among individuals or populations of the same species, with potential impacts on both species dynamics and evolution. However, this variability is hardly considered in empirical works, where a single dispersal function is considered to adequately reflect the species-specific dispersal ability, suggesting thereby that within-species variation is negligible as regard to inter-specific differences in dispersal abilities. We propose here an original method to make the comparison of intra- and inter-specific variability in dispersal, by decomposing the diversity of that trait along a phylogeny of closely related species. We used as test group European butterflies that are classic study organisms in spatial ecology. We apply the analysis separately to eight metrics that reflect the dispersal propensity, the dispersal ability or the dispersal efficiency of populations and species. At the inter-specific level, only the dispersal ability showed the signature of a phylogenetic signal while neither the dispersal propensity nor the dispersal efficiency did. At the within-species level, the partitioning of dispersal diversity showed that dispersal was variable or highly variable among populations: intra-specific variability represented from 11% to 133% of inter-specific variability in dispersal metrics. This finding shows that dispersal variation is far from negligible in the wild. Understanding the processes behind this high within-species variation should allow us to properly account for dispersal in demographic models. Accordingly, to encompass the within species variability in life histories the use of more than one value per trait per species should be encouraged in the construction of databases aiming at being sources for modelling purposes.     

Spatial genetic structure in two congeneric epiphytes with different dispersal strategies analysed by three different methods

Three different approaches were used to assess the kinship structure of two epiphytic bryophytes, Orthotrichum speciosum and O. obtusifolium, that have different dispersal strategies. The two species were sampled in a 200 ha landscape where species occurrence and host trees had been mapped previously. Local environmental conditions at sampled trees were recorded and kinship between individuals was calculated based on amplified fragment length polymorphism (AFLP)-marker data. We did not detect any association between AFLP-markers and investigated environmental conditions. In both species, significant kinship coefficients were found between individuals up to 300-350 m apart which shows that both species have a restricted dispersal range. The spatial kinship structure was detected with both autocorrelation analysis and generalized additive models (GAMs), but linear regression failed to detect any structure in O. speciosum. Although the dioecious O. obtusifolium is currently the more common species it may, none the less, due to its restricted dispersal range and reproduction mode, become threatened in the future by current silvicultural practices which enhance the distance between host trees and decrease their life span. Finally, GAMs seem most appropriate for analysing spatial genetic structure because the effects of local environmental conditions and spatial structure can be analysed simultaneously, no assumption of a parametric form between kinship coefficient and distance is required, and spatial data resolution is not lost in the arbitrary choice of distance classes characterizing autocorrelation analysis.     

The importance of temporal climate variability for spatial patterns in plant diversity

Spatial variation in absolute climatic conditions (means, maxima or minima) is widely acknowledged to play a fundamental role in controlling species diversity patterns. In contrast, while evidence is accumulating that variability around mean climatic conditions may also influence species coexistence and persistence, the importance of spatial variation in temporal climatic variability for species diversity is still largely unknown. We used a unique dataset capturing fine‐scale spatial heterogeneity in temperature variability across 2490 plots in southeast Australia to examine the comparative strength of absolute temperature and temperature variability in explaining spatial variation in plant diversity. Across all plots combined and in three of five forest types, temperature variability emerged as the better predictor of diversity. In all but one forest type, diversity also exhibited either a significant unimodal or positive linear correlation with temperature variability. This relationship is consistent with theory that predicts diversity will initially increase along a climate variability gradient due to temporal niche partitioning, but at an intermediary point, may decline as the risk of stochastic extinction exceeds competitive stabilization. These findings provide critical empirical evidence of a linkage between spatial variation in temporal climate variability and plant species diversity, and in light of changing climate variability regimes, highlight the need for ecologists to expand their purview beyond absolutes and averages.     

Biosystematic studies onCapsella bursa-pastoris (Brassicaceae): Enzyme polymorphism in natural populations

The genetic variability of natural populations ofCapsella bursapastoris in North- and Middle-Europe has been estimated by means of enzyme assays. Zymograms of 81 populations have been developed. 17 loci could be identified, and 8 of them can be heterozygous. Genetic variability is greater between populations than within. No correlation between actual population sizes and genetic heterogeneity could be detected. Some electromorphs shift their frequencies proportionally to increasing adversity of climatic conditions, some appear to be constant over the whole area, and others are characterized by an apparently irregular variation pattern. Marginal populations comprise a significantly higher proportion of heterozygous plants than central ones. Apart from this clinal variation pattern, a mosaic pattern, strongly related to habitat conditions, was observed: genetic heterogeneity is greater in more intensively disturbed sites. The pattern of genetic variation in natural populations ofCapsella bursa-pastoris is rather highly influenced by the breeding system.     

Male gametophytic selection in maize

Summary There is evidence that male gametophyte selection is a widespread phenomenon in higher plants. The pollen tube growth rate is one of the main components of gametophyte selective value; genetic variability for this trait, due to the effect of single genes or to quantitative variation, has been described in maize. However, indication of gametophytic selection has been indirectly obtained; its effect was revealed by the positive relation observed between gametophyte competitive ability and sporophyte metrical traits.This paper considers the results of selection applied to gametophyte populations produced from single plants. The competitive ability of the lines was evaluated in comparison with that of a standard line by means of the pollen mixture technique. Sporophytic traits were measured in the hybrid progeny obtained by crossing selected S₃ and S₄ families with an unrelated single cross and an inbred line. Gametophyte selection produced inbred lines with high gametophyte competitive ability. In view of the selection procedure adopted, this result was interpreted as an indication of haploid expression of genes involved in the control of pollen tube growth. Moreover, this gametophytic trait was positively correlated with sporophytic traits (seedling weight, kernel weight and root tip growth in vitro), indicating that both groups of characters have a common genetic basis.     

Arsenic hyperaccumulation in gametophytes of Pteris vittata. A new model system for analysis of arsenic hyperaccumulation

The sporophyte of the fern Pteris vittata is known to hyperaccumulate arsenic (As) in its fronds to >1% of its dry weight. Hyperaccumulation of As by plants has been identified as a valuable trait for the development of a practical phytoremediation processes for removal of this potentially toxic trace element from the environment. However, because the sporophyte of P. vittata is a slow growing perennial plant, with a large genome and no developed genetics tools, it is not ideal for investigations into the basic mechanisms underlying As hyperaccumulation in plants. However, like other homosporous ferns, P. vittata produces and releases abundant haploid spores from the parent sporophyte plant which upon germination develop as free-living, autotrophic haploid gametophyte consisting of a small (<1 mm) single-layered sheet of cells. Its small size, rapid growth rate, ease of culture, and haploid genome make the gametophyte a potentially ideal system for the application of both forward and reverse genetics for the study of As hyperaccumulation. Here we report that gametophytes of P. vittata hyperaccumulate As in a similar manner to that previously observed in the sporophyte. Gametophytes are able to grow normally in medium containing 20 mm arsenate and accumulate >2.5% of their dry weight as As. This contrasts with gametophytes of the related nonaccumulating fern Ceratopteris richardii, which die at even low (0.1 mm) As concentrations. Interestingly, gametophytes of the related As accumulator Pityrogramma calomelanos appear to tolerate and accumulate As to intermediate levels compared to P. vittata and C. richardii. Analysis of gametophyte populations from 40 different P. vittata sporophyte plants collected at different sites in Florida also revealed the existence of natural variability in As tolerance but not accumulation. Such observations should open the door to the application of new and powerful genetic tools for the dissection of the molecular mechanisms involved in As hyperaccumulation in P. vittata using gametophytes as an easily manipulated model system.     

On migration load of seeds and pollen grains in a local population

We have extended Wright's model of migration load to hermaphrodite plants showing variation at a single locus with two alleles. The model incorporates independent migration of seeds and pollen grains, the selection at both the haploid gametophyte and the diploid sporophyte stages, and a mixed mating system. The analytical relations between migration load and migration rate of seeds and pollen grains are explicitly formulated. The results show that under certain conditions, seed flow can have a more effect on migration load than pollen flow. Pollen selection at the gametophyte stage cannot substantially affect the migration load at the sporophyte stage. Selection at the diploid sporophyte stage is critical in determining the migration load of pollen grains. The relative migration loads of pollen versus seeds can be approximately estimated in predominantly outcrossing populations by the ratio of pollen flow to twice the seed flow, when the selection coefficient (s(T)) is greater than, or approximately equal to, the migration rate (m).     

Genetic structure of Polytrichum formosum in relation to the breeding system as revealed by microsatellites

Microsatellite variation was determined for three Danish and three Dutch populations of the haploid moss species Polytrichum formosum to gain insight into the relative importance of sexual vs. asexual reproduction for the amount and structure of genetic variation. In general, low levels of microsatellite variation were observed within this species. Even when estimated for polymorphic loci only, the levels of microsatellite variability (P=90.6, A=4.3 and H_S=0.468) within populations were on average lower than those reported for most other plant species. In contrast, genotypic diversity was high within each of the examined populations, indicating that sexual reproduction is a very important determinant of the genetic structure of P. formosum within populations. In agreement with previous findings for allozyme data, no significant genetic differentiation (F_ST=0.028, R_ST=0.015) was observed neither between populations nor between regions approximately 450 km apart (Denmark vs. the Netherlands). These low levels of population differentiation observed for both types of genetic markers are probably best explained by a high level of effective spore dispersal (gene flow) between populations. Therefore, also on a large geographical scale sexual reproduction is the most important determinant of the genetic structure of P. formosum, despite the high potential to reproduce clonally.     

Life history variation in gametophyte populations of the moss Ceratodon purpureus (Ditrichaceae)

The life cycles of mosses and other bryophytes are unique among land plants in that the haploid gametophyte stage is free-living and the diploid sporophyte stage is ephemeral and completes its development attached to the maternal gametophyte. Despite predictions that populations of haploids might contain low levels of genetic variation, moss populations are characterized by substantial variation at isozyme loci. The extent to which this is indicative of ecologically important life history variation is, however, largely unknown. Gametophyte plants from two populations of the moss Ceratodon purpureus were grown from single-spore isolates in order to assess variation in growth rates, biomass accumulation, and reproductive output. The data were analyzed using a nested analysis of variance, with haploid sib families (gametophytes derived from the same sporophyte) nested within populations. High levels of life history variation were observed within both populations, and the populations differed significantly in both growth and reproductive characteristics. Overall gametophytic sex ratios did not depart significantly from 1:1 within either population, but there was significant variation among families in both populations for progeny sex ratio. Some families produced predominantly male gametophytes, while others yielded predominantly females. Because C. purpureus has a chromosomal mechanism of sex determination, these observations suggest differential (but unpredictable) germination of male and female spores. Life history observations showed that male and female gametophytes are dimorphic in size, maturation rates, and reproductive output.