Selfing versus outcrossing in the androdioecious clam shrimp,Eulimnadia texana (Crustacea,Conchostraca)

The clam shrimpEulimnadia texana is an androdioecious crustacean in which hermaphrodites may self fertilize or outcross with males but cannot outcross with other hermaphrodites. Outcrossing is maintained within most populations of this species despite the high genetic cost of sex, suggesting that compensating factors provide an advantage to outcrossing. We hypothesized that one such benefit would be the production of larger clutch sizes resulting from outcrossed matings. To test this prediction, we recorded the body sizes and clutch sizes of hermaphrodites which mated via selfing or bia outcrossing. Clutch sizes showed significant, almost exponential, increases as body size increased in both selfing and outcrossing hernmaphrodites. The rate of this increase was the same for both groups, and there was no significant difference in clutch size when body size was controlled for between the two fertilization types.     

Sex chromosome evolution in the clam shrimp,Eulimnadia texana

Chromosomes that determine sex are predicted to evolve differently than autosomes: a lack of recombination on one of the two sex chromosomes is predicted to allow an accumulation of deleterious alleles that eventually leads to reduced functionality and potential physical degradation of the nonrecombining chromosome. Because these changes should occur at an elevated evolutionary rate, it is difficult to find appropriate species in which to test these evolutionary predictions. The unique genetic sex‐determining mechanism of the crustacean Eulimnadia texana prevents major chromosome degeneration because of expression of both ‘proto‐sex’ (i.e. early stage of development) chromosomes in homozygous form (ZZ and WW). Herein, we exploit this unique genetic system to examine the predicted accumulation of deleterious alleles by comparing both homogametic sexual types to their heterogametic counterpart. We report differences in crossing over in a sex‐linked region in the ZW hermaphrodites (～ 3%) relative to the ZZ males (～ 21%), indicative of cross‐over suppression in the ZW hermaphrodites. Additionally, we report that both ZZ and WW genotypes have reduced fitness relative to ZW hermaphrodites, which is consistent with the prediction of harboured recessive mutations embedded on both the Z and the W chromosomes. These results suggest that the proto‐sex chromosomes in E. texana accumulate recessive deleterious alleles. We hypothesize that recessive deleterious alleles of large effect cannot accumulate because of expression in both ZZ and WW individuals, keeping both chromosomes from losing significant function.     

Is sexual selection operating in the androdioecious clam shrimp,Eulimnadia texana (Crustacea: Conchostraca)?

Experiments were performed to document the existence of intersexual or intrasexual selection in the clam shrimp,Eulimnadia texana. Individuals within this species are either males or hermaphrodites. Hermaphrodites can self their own eggs or outcross with a male, but they cannot outcross with other hermaphrodites. Theoretical considerations suggest that both intrasexual and intersexual selection could be occurring on the part of the hermaphrodites and the males. When males were given a choice between two non-gravid hermaphrodites of different sizes, they did not exhibit a mating preference based upon size. When two males of different sizes were isolated with a single nongravid receptive hermaphrodite, the hermaphrodite showed no preference between the two males. There was evidence, however, of male-male competition for receptive hermaphrodites and of mate guarding on the part of the males. During aggressive encounters between twp males, the larger of the two had a significant advantage over the smaller, and larger males were always the victors hermaphrodite takeovers occurred as a result of male-male conflict. Hermaphrodites appear to control the mating process both by struglling with males when they are not receptive to them and by selfing in the presence of males. This suggests that hermaphrodites withhold receptivity cues from males, or produce non-receptivity cues, when they are going to self. Though hermaphrodites do not appear to select males based upon size, they make a selection between selfing and outcrossing by controlling the use of receptivity signals.     

Notes on the life history of the clam shrimp,Eulimnadia texana

Several life history measures (growth rate, egg production, molt frequency, age at maturity and lifespan) were measured on several clam shrimp hermaphrodites (Eulimnadia texana Packard) grown in a laboratory setting under optimal growth conditions. Growth rates were high early in life, and then dropped dramatically when egg production began (day 5–6). Early egg production was low, and increased until approximately day 7, after which production leveled off for several days. Reproductive senescence was noted after day 17, with clutch sizes continuously dropping until death. Average molts per day was approximately 1.1, and molting seemed to be more closely associated with egg production than with growth. Growth and egg production were negatively correlated, indicating a possible trade-off between these two traits. No other trade-offs were detected. These shrimp show typical early-colonist life history traits, displaying high initial growth, early reproduction at a high rate, and then early senescence and death. This revised version was published online in August 2006 with corrections to the Cover Date.     

Production of intersexes and the evolution of androdioecy in the clam shrimp Eulimnadia texana (Crustacea,Branchiopoda, Spinicaudata)

Summary

The production of low numbers of offspring that exhibit a mixture of male and female traits (termed “intersexes”) is commonly reported for crustaceans. The production of intersexes has been ascribed to both genetic and non-genetic (e.g., parasitic infections and environmental pollutants) causes. Herein we report on two observed types of intersexes in the clam shrimp Eulimnadia texana: (1) a “morphological” intersex, possessing secondary male characteristics (e.g., claspers) and an eggproducing gonad, and (2) a “gonadal” intersex, possessing primarily male traits (e.g., male secondary sexual characters and male gamete production) but also producing low levels of abortive female gametes. We propose that these intersexes are likely the products of low frequencies of crossing over between the sex determining chromosomes that result in the array of observed mixed sexual phenotypes. Additionally, we suggest that the low-level production of intersexes, combined with the ephemeral nature of the habitats occupied by these shrimp, may explain the preponderance of androdioecy (mixtures of males and hermaphrodites) found in these clam shrimp, and possibly branchiopods more generally.     

Mating behavior and time budget of an androdioecious crustacean,Eulimnadia texana (Crustacea: Conchostraca)

The clam shrimp,Eulimnadia texana (Crustacea, Conchostraca), is found in freshwater ephemeral environments throughtout the United States. Individual clam shrimp of this species are either hermaphroditic or male, a relatively rare mating system for animals known as androdioecy. Comparison of sex ratios between four neighboring populations ofE. texana in Southern New Mexico showed wide variation in the ratio of males to hermaphrodites with males making up as much as 42% of some populations and not occurring at all within others. Since little is known about the behavior of this species, an ethogram and time budget were prepared based on observations of laboratory populations. Males attempt to clasp hermaphrodites prior to mating. Precopulatory mate guarding occurs in this species. Outcrossing generally occurs during mate guarding and after the hermaphrodite molts. Hermaphrodites, however, seem to control the mating process. Successful mating by males never occured if the hermaphrodite struggled with him; hermaphrodite will self in the presence of males.     

Maintenance of androdioecy in the freshwater shrimp Eulimnadia texana: sexual encounter rates and outcrossing success

The clam shrimp Eulimnadia texana has a rare mating system known as androdioecy, in which males and hermaphrodites cooccur butthere are no pure females. In this species, reproduction takesplace by outcrossing between males and hermaphrodites, or byselfing within a hermaphrodite; this system provides a uniqueopportunity to examine the adaptive significance of out-crossingand selfing in animals. Our study examined mating behavior in hermaphrodites and males from two populations to understandthe propensity of these shrimp to mate and to estimate a parameterof a model developed by Otto et al. (American Naturalist 141:329-337),which predicts the conditions for stability of the mixed matingsystem in E. texana. Here we present evidence that mating frequencyis environmentally sensitive, with greater numbers of encountersand matings per male when males are rare and in younger males.However, the effects of shrimp density, relative male frequency,and shrimp age interact in a complex way to determine malemating success. Overall, mating frequency was determined bya combination of encounter rates between the sexes and theproportion of encounters resulting in mating. The mating rateswere then used to estimate one of four parameters of the Ottoet al. model, and these estimates were combined with previousestimates of the other three parameters to examine the fitof the predicted to the observed sex ratios in the two populations.     

Cyst development in the conchostracan shrimp,Eulimnadia texana (Crustacea: Spinicaudata)

The fertilized egg (or cyst) of branchiopods is a highly resistant stage in the life cycle of these aquatic crustaceans. Previous examinations of these cysts have determined that early embryonic development arrests at a late blastula stage, resulting in a small, crescent-shaped body within the egg shell of these shrimp. Herein, we examine the early development of these embryos by sectioning eggs in the ovotestis, brood chamber, and several time periods after exit from the brood chamber in the clam shrimp Eulimnadia texana Packard. The early sections find no evidence of internal fertilization in the ovotestis. Eggs in the ovotestis showed no signs of cell division, whereas eggs sectioned from the brood chamber were found to be undergoing early embryonic development. A number of empty egg shells and the lack of unfertilized eggs in the brood chamber suggested that egg yolks quickly degrade after egg extrusion from the ovotestis. Cysts that were allowed to develop for 24, 48, 72, and 96 h, 1 week and 1.5 years were sectioned, and embryonic development did not change after the 48 h time period. Thus, embryos appear to arrest development somewhere between 24 and 48 h after exiting the brood chamber.     

A field test of a model for the stability of androdioecy in the freshwater shrimp,Eulimnadia texana

The evolution of hermaphroditism from dioecy is a poorly studied transition. Androdioecy (the coexistence of males and hermaphrodites) has been suggested as an intermediate step in this evolutionary transition or could be a stable reproductive mode. Freshwater crustaceans in the genus Eulimnadia have reproduced via androdioecy for 24+ million years and thus are excellent organisms to test models of the stability of androdioecy. Two related models that allow for the stable maintenance of males and hermaphrodites rely on the counterbalancing of three life history parameters. We tested these models in the field over three field seasons and compared the results to previous laboratory estimates of these three parameters. Male and hermaphroditic ratios within years were not well predicted using either the simpler original model or a version of this model updated to account for differences between hermaphroditic types (‘monogenic’ and ‘amphigenic’ hermaphrodites). Using parameter estimates of the previous year to predict the next year's sex ratios revealed a much better fit to the original relative to the updated version of the model. Therefore, counter to expectations, accounting for differences between the two hermaphroditic types did not improve the fit of these models. At the moment, we lack strong evidence that the long‐term maintenance of androdioecy in these crustaceans is the result of a balancing of life history parameters; other factors, such as metapopulation dynamics or evolutionary constraints, may better explain the 24+ million year maintenance of androdioecy in clam shrimp.     

Maintenance of androdioecy in the freshwater shrimp, Eulimnadia texana: estimates of inbreeding depression in two populations

Androdioecy is an uncommon form of reproduction in which males coexist with hermaphrodites. Androdioecy is thought to be difficult to evolve in species that regularly inbreed. The freshwater shrimp Eulimnadia texana has recently been described as both androdioecious and highly selfing and is thus anomalous. Inbreeding depression is one factor that may maintain males in these populations. Here we examine the extent of "late" inbreeding depression (after sexual maturity) in these clam shrimp using two tests: (1) comparing the fitness of shrimp varying in their levels of individual heterozygosity from two natural populations that differ in overall genetic diversity; and (2) specifically outcrossing and selfing shrimp from these same populations and comparing fitness of the resulting offspring. The effects of inbreeding differed within each population. In the more genetically diverse population, fecundity, size, and mortality were significantly reduced in inbred shrimp. In the less genetically diverse population, none of the fitness measures was significantly lowered in selfed shrimp. Combining estimates of early inbreeding depression from a previous study with current estimates of late inbreeding depression suggests that inbreeding depression is substantial (delta = 0.68) in the more diverse population and somewhat lower (delta = 0.50) in the less diverse population. However, given that males have higher mortality rates than hermaphrodites, neither estimate of inbreeding depression is large enough to account for the maintenance of males in either population by inbreeding depression alone. Thus, the stability of androdioecy in this system is likely only if hermaphrodites are unable to self-fertilize many of their own eggs when not mated to a male or if male mating success is generally high (or at least high when males are rare). Patterns of fitness responses in the two populations were consistent with the hypothesis that inbreeding depression is caused by partially recessive deleterious alleles, although a formal test of this hypothesis still needs to be conducted.     

Relative fitness of two hermaphroditic mating types in the androdioecious clam shrimp,Eulimnadia texana

Androdioecy (populations of males and hermaphrodites) is a rare reproductive form, being described from only a handful of plants and animals. One of these is the shrimp Eulimnadia texana, which has populations comprised of three mating types: two hermaphroditic types (monogenics and amphigenics) and males. In a recent study, the amphigenic hermaphrodites were found to be in greater abundance than that predicted from a model of this mating system. Herein, we compare the relative fitness of offspring from amphigenic and monogenic siblings, attempting to understand the greater relative abundance of the former. Populations started with offspring from selfed monogenic hermaphrodites had a net reproductive rate (R) 87% that of offspring from their amphigenic siblings. Additionally, within populations of amphigenic offspring (which included males, monogenics and amphigenics), amphigenics survived longer than monogenics. These differences help to explain the increased relative abundance of amphigenics in natural populations, but amphigenics continue to be more abundant than expected.     

Levels of inbreeding depression over seven generations of selfing in the androdioecious clam shrimp, Eulimnadia texana

Androdioecy (mixtures of males and hermaphrodites) is a rare mating system in both plants and animals. Theory suggests that high levels of inbreeding depression can maintain males in androdioecious populations if hermaphrodites commonly self-fertilize. However, if inbreeding depression (delta) can be 'purged' from selfing populations, maintaining males is more difficult. In the androdioecious clam shrimp, Eulimnadia texana, delta is estimated to be as high as 0.7. Previous work suggests that this high level is maintained in the face of high levels of inbreeding due to an associative overdominance of fitness-related loci with the sex-determining locus. Such associative overdominance would make purging of inbreeding depression difficult to impossible. The current experiment was designed to determine if delta can be purged in these shrimp by tracking fitness across seven generations in selfing and outcrossing treatments. Evidence of purging was found in one of four populations, but the remaining populations demonstrated a consistent pattern of delta across generations. Although the experimental design allowed ample opportunity for purging, the majority of populations were unable to purge their genetic load. Therefore, delta in this species is likely due to associative overdominance caused by deleterious recessive alleles linked to the sex determining locus.     

Barriers to outcrossing success in the primarily self-fertilizing clam shrimp, Eulimnadia texana (Crustacea, Branchiopoda)

Abstract. The expected proportion of males in androdioecious populations (those comprised of males and hermaphrodites) largely depends on the fertilization opportunities of males. If male mating opportunities are low due to restricted access to hermaphroditic eggs, then populations will be hermaphrodite-biased. Hermaphrodites have two mechanisms available to limit male mating success: (1) pre-mating barriers to outcrossing, in which hermaphrodites choose not to pair with males and (2) post-mating barriers to outcrossing, in which hermaphrodite sperm has greater access to eggs than male sperm. In this study, we measured male mating success in the androdioecious clam shrimp Eulimnadia texana when pre-mating barriers to outcrossing were removed. These branchiopod crustaceans are small (5–8 mm), filter feeders that live in ephemeral pools in the deserts of the southwestern United States. Using genetic markers, we measured male mating success in laboratory experiments in two populations of these shrimp. We correlated mating success with clasping time, clasping during egg transfer, and male thrusting during egg transfer. Males fertilized an average of 24–40% of the hermaphrodites' eggs. Outcrossing success was positively correlated with clasping duration, and was nearly an order of magnitude higher for males thrusting during egg transfer relative to thrusting at other times during pairing. Because these estimates of mating success were similar to previously reported estimates (in which both pre- and post-mating barriers to outcrossing were potentially important), we deduced that pre-mating barriers to outcrossing do not greatly decrease male outcrossing success in E. texana ; the low fertilization (25–50% of available eggs) by males is thus due to post-mating barrier(s) to outcrossing.     

Effects of dietary restriction on lifespan,growth, and reproduction of the clam shrimp Eulimnadia texana

Hydrobiologia - Diet has been linked to lifespan in a broad range of animals. In particular, pronounced caloric restriction has been associated with increased longevity. Herein, the relationship of...     

Ancient androdioecy in the freshwater crustacean Eulimnadia

Among the variety of reproductive mechanisms exhibited by living systems, one permutation--androdioecy (mixtures of males and hermaphrodites)--is distinguished by its rarity. Models of mating system evolution predict that androdioecy should be a brief stage between hermaphroditism and dioecy (separate males and females), or vice versa. Herein we report evidence of widespread and ancient androdioecy in crustaceans in the genus Eulimnadia, based on observations of over 33,000 shrimp from 36 locations from every continent except Antarctica. Using phylogenetic, biogeographical and palaeontological evidence, we infer that androdioecy in Eulimnadia has persisted for 24-180 million years and has been maintained through multiple speciation events. These results suggest that androdioecy is a highly successful aspect of the life history of these freshwater crustaceans, and has persisted for orders of magnitude longer than predicted by current models of this rare breeding system.     

Isolation and characterization of 13 polymorphic microsatellite loci from the clam shrimp Eulimnadia texana (Crustacea: Spinicaudata)

Thirteen polymorphic microsatellite loci were isolated and characterized from the clam shrimp Eulimnadia texana. In analyses of 20–50 individuals from two populations the number of alleles ranged from two to seven with observed heterozygosity ranging between 0.00 and 0.37. The low values for heterozygosity were not unexpected for a group characterized by its unusual androdioecious mating system, in which males compete with self‐compatible hermaphrodites for offspring production. These microsatellites are likely to be useful for further evolutionary investigations of this rare mating system in these crustaceans.     

Ultrastructure of the male gonad and male gametogenesis in the clam shrimp Eulimnadia texana (Crustacea, Branchiopoda, Spinicaudata)

Abstract. The ultrastructure of the male gonad of Eulimnadia texana (Branchiopoda, Spinicaudata) has been observed for the first time to investigate the sexuality of a well-studied case of androdioecy in the animal kingdom. The male gonad is a double structure located in the hemocoel throughout the entire body length on each side of the midgut. Male gametes originate from the wall and mature centripetally toward the lumen; the proliferative activity is very high and continuous and therefore the mature gonad is full of numerous germ cells. Inside the lumen several degenerative stages are found mixed with sperm cells and spermatids, the latter two being not easily distinguishable because of the slight differences between them. The evolutionary meaning of the degenerative process in E. texana male gametes is difficult to explain, and we propose some hypotheses about its possible role or cause in the studied population: (a) to help build spermatophores, (b) to act as a trophic component for viable sperm, (c) as a manifestation of inbreeding depression, and/or (d) to regulate the number of sperm cells.     

Floral development in the androdioecious tree Tapiscia sinensis: Implications for the evolution to androdioecy

The evolutionary pathway between hermaphroditism and dioecy (females and males in a single population) draws widespread interests, and androdioecy (bisexuals and males in a single population) is rarely achieved as an intermediate state between the two breeding systems. Flower bud differentiations in the pistils of hermaphrodites and the pistillodes of males in androdioecious Tapiscia sinensis Oliv. are investigated by routine paraffin section technology, light microscopy, and scanning electron microscopy. A phylogenetic approach is used to analyze the origin of androdioecy. In T. sinensis, hermaphroditic flowers (HF) and male flowers (MF) experienced a similar development pattern in early flower bud differentiation, including the initiation of tepals and stamens. However, the carpel differentiation of MF and HF proceed in different patterns. In HF, the central zone bulges out and produces a ring meristem on which two to three carpel primordia emerge, which eventually developed into a normal pistil with a stigma, a style, and an ovary. However, in most MF, vestigial pistils are stem‐like (type I), and very few have an empty ovary (type II) or a sterile ovule (type III). Moreover, the evolution of sexual systems within the Huerteales indicates that hermaphroditism is the primitive character of T. sinensis. Tapiscia sinensis shows different degrees of reduction between male flowers and bisexual ones in the evolution to dioecy. Functional androdioecy originated from a hermaphroditic ancestor in T. sinensis and, as an intermediate sexual system, involves evolution from hermaphrodites to dioecy.     

A genetic comparison of two species of clam shrimp in the genus Eulimnadia: An electrophoretic approach

Herein we report the first genetic comparison among species in the genus Eulimnadia. Multilocus genotypic patterns (using six allozyme loci) were compared for a total of 2277 clam shrimp from nine populations from Arizona and New Mexico. Seven of these populations were morphologically typed as Eulimnadia texana Packard and two were typed as Eulimnadia diversa Mattox. All populations were hermaphrodite-biased, and highly inbred (inbreeding coefficients ranging from 0.33 to 0.98). Genetic distances showed the two species to be within the range described for other arthropods. One of the two E. diversa populations appeared to be a hybrid between E. texana and E. diversa, showing electrophoretic patterns similar to both species, although morphologically, they were typed as E. diversa. A phenogram (generated using coancestry distances and a neighbor joining algorithm) placed this hybrid population half-way between these two species, and a breakdown of individuals within this hybrid population (based on allozyme scores) indicated individuals very similar to the second E. diversa population, and two groups of apparent hybrid individuals. Therefore, the distinction between these two species is questionable due to their apparent hybridization in this area of Arizona. Genetic population structuring was noted among the seven E. texana pools. Estimated migration rates were less than one migrant per generation. Even in the geographically close pools in New Mexico, which were separated by only hundreds of meters, significant sub-structuring was noted, and estimates of migration rate were less than two migrants per generation.     

Evolutionary transitions among dioecy,androdioecy and hermaphroditism in limnadiid clam shrimp (Branchiopoda: Spinicaudata)

Examinations of breeding system transitions have primarily concentrated on the transition from hermaphroditism to dioecy, likely because of the preponderance of this transition within flowering plants. Fewer studies have considered the reverse transition: dioecy to hermaphroditism. A fruitful approach to studying this latter transition can be sought by studying clades in which transitions between dioecy and hermaphroditism have occurred multiple times. Freshwater crustaceans in the family Limnadiidae comprise dioecious, hermaphroditic and androdioecious (males + hermaphrodites) species, and thus this family represents an excellent model system for the assessment of the evolutionary transitions between these related breeding systems. Herein we report a phylogenetic assessment of breeding system transitions within the family using a total evidence comparative approach. We find that dioecy is the ancestral breeding system for the Limnadiidae and that a minimum of two independent transitions from dioecy to hermaphroditism occurred within this family, leading to (1) a Holarctic, all‐hermaphrodite species, Limnadia lenticularis and (2) mixtures of hermaphrodites and males in the genus Eulimnadia. Both hermaphroditic derivatives are essentially females with only a small amount of energy allocated to male function. Within Eulimnadia, we find several all‐hermaphrodite populations/species that have been independently derived at least twice from androdioecious progenitors within this genus. We discuss two adaptive (based on the notion of ‘reproductive assurance’) and one nonadaptive explanations for the derivation of all‐hermaphroditism from androdioecy. We propose that L. lenticularis likely represents an all‐hermaphrodite species that was derived from an androdioecious ancestor, much like the all‐hermaphrodite populations derived from androdioecy currently observed within the Eulimnadia. Finally, we note that the proposed hypotheses for the dioecy to hermaphroditism transition are unable to explain the derivation of a fully functional, outcrossing hermaphroditic species from a dioecious progenitor.