Hermaphroditism: What's not to like?

Hermaphroditism is rare and phylogenically in decline among animal species. The evolutionary basis for this development is not well understood. This paper focusses on self-incompatible simultaneous hermaphroditism in animals. It proposes that such hermaphroditism is not stable in sufficiently heterogeneous populations, suggesting a possible reason for why hermaphroditism is rare among evolved animal species. The argument turns on the Bateman principle, namely that male reproductive success (RS) is limited by partner availability, while female RS is not. We show that: low-quality individuals do better if female; secondary sexual differentiation may be important for understanding the existence of males; and that hermaphroditic mating is reciprocal. Reciprocity may be key to understanding promiscuity and attendant phenomena such as cryptic female choice, sperm competition and love darts-common features of hermaphroditic mating. We also argue that hermaphrodites are especially vulnerable to male violence, suggesting a reason for the rarity of trioecy. Finally, we propose that external fertilization, and the scope for streaking, may be one reason fish are the only simultaneously hermaphroditic vertebrates.     

Sex determination in the androdioecious plant Datisca glomerata and its dioecious sister species D. cannabina.

Datisca glomerata is an androdioecious plant species containing male and hermaphroditic individuals. Molecular markers and crossing data suggest that, in both D. glomerata and its dioecious sister species D. cannabina, sex is determined by a single nuclear locus, at which maleness is dominant. Supporting this conclusion, an amplified fragment length polymorphism (AFLP) is heterozygous in males and homozygous recessive in hermaphrodites in three populations of the androdioecious species. Additionally, hermaphrodite x male crosses produced 1:1 sex ratios, while hermaphrodite x hermaphrodite crosses produced almost entirely hermaphroditic offspring. No perfectly sex-linked marker was found in the dioecious species, but all markers associated with sex mapped to a single linkage group and were heterozygous in the male parent. There was no sex-ratio heterogeneity among crosses within D. cannabina collections, but males from one collection produced highly biased sex ratios (94% females), suggesting that there may be sex-linked meiotic drive or a cytoplasmic sex-ratio factor. Interspecific crosses produced only male and female offspring, but no hermaphrodites, suggesting that hermaphroditism is recessive to femaleness. This comparative approach suggests that the hermaphrodite form arose in a dioecious population from a recessive mutation that allowed females to produce pollen.     

Sexual selection and reproductive success in hermaphroditic seabasses

Mating behavior in simultaneously hermaphroditic seabasses hasbeen often cited as an example of cooperation among unrelatedconspecifics. The predominant mating behavior in this groupinvolves egg trading, where individuals reciprocally fertilizeparcels of eggs from a partner. Egg trading has been suggestedas a good example of a tit-for-tat cooperative mating strategy.Although simultaneous hermaphroditic fishes are often held upas strong examples of cooperation in mating behavior, a closerexamination reveals significant sexual selection and sexualconflict between male and female roles among individuals. Inthe 7 species where data exist, there is a significant increasein male reproductive success with individual size, and in allbut 1 species success through male function increases fasterthan reproductive success through female function. Despite thismale-size advantage in simultaneous hermaphrodites, most speciesmaintain their hermaphroditism for their entire life, and theincreased male allocation while engaging in biased forms ofreciprocation appear to increase the evolutionary stabilityof hermaphroditism in these species. Thus, egg-trading behavioris probably more complicated than was initially recognized,with individuals releasing different numbers of eggs in spawns,spawning at different rates as males and females, and partitioningmale effort between pair and alternative mating tactics. Thedepartures from equal reciprocity can probably be best understoodby including aspects of traditional mating-system theory, withindividuals increasing male mating success through a varietyof behavioral tactics.     

Sequential patterns of sex allocation in simultaneous hermaphrodites: do we need models that specifically incorporate this complexity?

Theoretical and empirical studies of sex allocation usually treat sequential and simultaneous hermaphroditism as distinct and disparate forms of allocation. However, the sexual patterns of numerous species have both sequential (e.g., size-based) and simultaneous components. In most cases, we have drawn from sex allocation theory developed for sequential hermaphrodites to explain ontogenetic changes in allocation and from theory developed for simultaneous hermaphrodites to explain the remaining aspects of these sexual patterns rather than develop a more integrated theory. Here I present the evolutionary stable solution (ESS) to a dynamic statevariable model that explicitly combines the effects of size and simultaneous allocation to male and female function in a dynamic game. The model structure and initial parameter values are based on the sexual pattern of the blue-banded goby, Lythrypnus dalli, a simultaneous hermaphrodite. I then compare the natural patterns of sex allocation in L. dalli with the predictions of the model and with those of a dynamic version of the size advantage model. The integrated model predicted variation in allocation, sex-specific size distributions, and seasonal sex ratio better than the sequential hermaphroditism model did. Indeed, the sequential model, using L. dalli parameter values, predicts a dioecious rather than sequentially hermaphroditic allocation pattern. The comparison of these two models illustrates the disadvantage of drawing from two bodies of theory without a formal integrated framework. Furthermore, the comparison focuses attention on the role of costs of reallocation in the evolution of mixed (or intermediate) sexual patterns.     

Sex allocation in a simultaneous hermaphrodite, the blue-banded goby (Lythrypnus dalli): the effects of body size and behavioral gender and the consequences for reproduction

Although species with both male and female sexual functionsare often dichotomized into simultaneous and sequential hermaphrodites,many simultaneous hermaphrodites also exhibit sequential changesin sex allocation. In a field experiment using one such species,the gobiid fish, Lythrypnus dalli, female-biased individualsreallocated to male function in relation to their relative bodysize: consistent with the sizeadvantage hypothesis, large femaleswere more likely to reallocate and large fish had the highestspawningrates. Individuals, despite internal allocation to bothsexual functions, adopted only one behavioral gender. Behavioralmales had higher reproductive rates than behavioral females,and laboratory experiments showed that females preferred tomate with large males. Behavioral males grew more rapidly anddid notdiffer from behavioral females in survivorship. In addition,individuals who adopted male behavior but did not receive eggsin their nests maintained high levels of female tissue, whereasmales that received eggs did not. Laboratory experiments showedthat, unlike most hermaphroditic animals, L. dalli canchangeallocation either from ‘female’ to ‘male’or from ‘male’ to ‘female’. Thus, L.dalli shares haracteristics of both sequential and simultaneoushermaphrodites. Simultaneous hermaphroditism maybe maintained,in this species, to facilitate rapid sex change from femaleto male and to retain flexibility o that unsuccessful malescan revert to reproduction as females.     

Do tiny males grow up? Sperm competition and optimal resource allocation schedule of dwarf males of barnacles

Barnacles, marine crustaceans, have three sexual patterns: simultaneous hermaphroditism, dioecy and androdioecy. In dioecy and androdioecy, large individuals (females and hermaphrodites, respectively) are attached by dwarf males. Depending on species, some dwarf males grow up, others do not in their life time. To investigate which environmental conditions affect growth patterns of dwarf males of barnacles, we investigate the evolutionarily stable life history strategy of dwarf males using Pontryagin's maximum principle. Sperm competition among dwarf males and that among dwarf males and large hermaphrodites is taken into account. Dwarf males grow up in food-rich environments, while they do not grow at all in food-poor environments. ESS of the resource allocation schedule between reproduction and growth follows an "intermediate growth strategy" (simultaneous growth and reproduction) for dioecious species, in which sperm competition is not severe. On the other hand, it approaches "bang-bang control" (switching from allocating all resources toward growth then to reproduction), as sperm competition against surrounding large hermaphrodites becomes severe in androdioecious species.     

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.     

Intersexual conflict in androdioecious clam shrimp: Do androdioecious hermaphrodites evolve to avoid mating with males?

A recent sexual conflict model posits that a form of intersexual conflict may explain the persistence of males in androdioecious (males + hermaphrodites) populations of animals that are being selected to transition from dioecious (gonochoristic) mating to self‐compatible hermaphroditism. During the evolutionary spread of a self‐compatible hermaphrodite to replace females, the selective pressures on males to outcross are in conflict with the selective pressures on hermaphrodites to self. According to this model, the unresolved conflict interferes with the evolutionary trajectory from dioecy to hermaphroditism, slowing or halting that transition and strengthening the otherwise “transitory” breeding system of androdioecy into a potentially stable breeding strategy. Herein, we assess this model using two dioecious and two androdioecious clam shrimp (freshwater crustaceans) to ask two questions: (1) Have hermaphrodites evolved so that males cannot effectively recognize them?; and (2) Do androdioecious hermaphrodites avoid males? Androdioecious males made more mistakes than dioecious males when guarding potential mates suggesting that androdioecious males were less effective at finding hermaphrodites than dioecious males were at finding females. Similarly, in a three‐chambered experiment, focal hermaphrodites chose to aggregate with their same sex, whereas focal dioecious males chose to aggregate with the alternate sex. Together, these two experiments support the sexual conflict model of the maintenance of androdioecy and suggest that hermaphrodites are indeed evolving to avoid and evade males.     

Sustainability of dioecious and hermaphrodite populations on a lattice

Sexual reproduction may be divided into two main categories: hermaphroditism and dioecy (Botany)/gonochorism (Zoology). Simultaneous hermaphrodites can function in both male and female roles whereas a dioecious/gonochorist population consists of distinct male and female individuals. Mean-field calculations, which ignore spatial aspects, suggest that self-incompatible hermaphrodites should have a twofold advantage over dioecious population when reproduction is limited by mating encounters. By use of stochastic spatial simulations we demonstrate that hermaphroditism has an even greater advantage when local interactions are considered. This result provides further support for the observation that hermaphroditism is associated with sedentary species, such as plants and animals with poor mate search efficiency. We also investigate the finite size effects associated with the well-known quadratic contact process.     

A novel sexual pattern in serranid fishes: simultaneous hermaphrodites and secondary males in Serranus fasciatus

Synopsis Gulf of California populations of Serranus fasciatus are composed of functional simultaneous hermaphrodites and males. This is the first serranid known to have this sexual pattern which is functionally intermediate between the typical serranid patterns of simultaneous and protogynous hermaphroditism. Males of S. fasciatus are derived from hermaphrodites by resorption of ovarian tissue and proliferation of the extant testicular band. Distinct sexual roles are evident in spawning events. Hermaphrodites gain female function by pair spawning with males (124 and 125 observed spawns) and rarely with other hermaphrodites (1 of 125). Hermaphrodites gain male function by sneak spawning (9 of 125) and rarely by pair spawning with other hermaphrodites. Males exclusively pair spawned with hermaphrodites. Despite its unusual sexual pattern, S. fasciatus appears allied with other Serranus species based on similarities in gonad morphology.     

Evolution: vertebrate reproductive strategies get mixed up

The mangrove killifish is the only vertebrate known to have a mixed-mating strategy, where hermaphrodites reproduce by either self-fertilisation or cross-breeding. New molecular evidence from this species reveals that occasional cross-breeding between common hermaphroditic individuals and rare pure males results in an injection of genetic variation into otherwise highly homozygous 'clonal' lineages.     

Mate-search efficiency can determine the evolution of separate sexes and the stability of hermaphroditism in animals

Limited availability of mating partners has been proposed as an explanation for the occurrence of simultaneous hermaphroditism in animals with pair mating. When low population density or low mobility of a species limits the number of potential mates, simultaneous hermaphrodites may have a selective advantage because, first, they are able to adjust the allocation of resources between male and female functions in order to maximize fitness; second, in a hermaphroditic population the likelihood of meeting a partner is higher because all individuals are potential mates; and, third, in the absence of mating partners, many simultaneously hermaphroditic animals have the option of reproducing through self-fertilization. Recognizing that mate availability is central to the existing theory of hermaphroditism in animals, it is important to examine the effects of mate search on predictions of the stability of hermaphroditism. Many hermaphroditic animals can increase the number of potential mates they contact by active searching. However, since mate search has costs in terms of time and energy, the increased number of potential mates will be traded off against the amount of resources that can be allocated to the production of gametes. We explore the consequences of this trade-off to the evolution of mating strategies and to the selective advantage of self-fertilization. We show that in low and moderate population densities, poor mate-search efficiency and high costs of searching stabilize hermaphroditism and bias sex allocation toward female function. In addition, in very low population densities, there is strong selective advantage for self-fertilization, but this advantage decreases considerably in species with high mate-search efficiency. Most important, however, we present a novel evolutionary prediction: when mate search is efficient, disruptive frequency-dependent selection on time allocation to mate search leads to the evolution of searching and nonsearching phenotypes and, ultimately, to the evolution of males and females.     

Sex Allocation in Lythrypnus (Gobiidae): Variations on a Hermaphroditic Theme

The gonadal sexual patterns of three Atlantic/Caribbean Lythrypnus (L. nesiotes, L. phorellus, L. spilus) are described, based on histological examination, and compared with previously described sexual patterns of L. dalli and L. zebra, from the eastern Pacific. Overall, there was striking similarity in the sexual patterns of all the species; each exhibited simultaneous hermaphroditism with a high degree of variability in allocation to male tissue among individuals. Among the five species whose sexual patterns are described, at least three distinct sexual patterns were identified. Lythrypnus dalli has an allocation pattern distinct from the other species, with populations consisting of primarily pure females, strongly female-biased hermaphrodites and pure males. Lythrypnus nesiotes was also found to be distinct from the other species, consisting primarily of pure females and strongly female-biased hermaphrodites and male-biased individuals (mean allocation to male tissue = 75%). In contrast to L. dalli, L. nesiotes had no pure males. Indeed, no other species examined included pure males. L. phorellus, L. spilus and L. zebra had much more intermediate allocation patterns than either L. dalli or L. nesiotes. Most individuals were female biased (<10% male tissue), and there were significant numbers of individuals with intermediate allocation (10–90% male). L. zebra differed somewhat from L. phorellus and L. spilus in that more strongly male-biased individuals were common in the sample (i.e., ranging in allocation from 90 to 98% male); this difference was not statistically compelling. This marked variation in allocation among closely related species suggests Lythrypnus is a useful model system to explore factors affecting allocation. For example, models developed for simultaneous hermaphrodites and appropriate for Lythrypnus predict that pure sexed individuals will be more common at higher density (e.g., because mating success is more certain). The observed interspecific variation supports this prediction – both L. dalli and L. nesiotes occur at high density and have the most extreme allocation patterns, whereas L. phorellus, L. spilus, and L. zebra occur at lower density and have more intermediate allocation patterns.     

Simultaneous hermaphroditism; cost and benefit

Current theories explain simultaneous hermaphroditism by the advantage it gives to organisms which are widely dispersed or sluggish, resulting in a low frequency of reproductive contacts. It is difficult to see why hermaphroditism is not more widespread unless there is some counterbalancing disadvantage.It is suggested that hermaphrodites suffer an energetic cost because they maintain two reproductive systems and a cost due to the reduced number or viability of offspring which may result from accidental self-fertilization. These costs will result in a disadvantage to hermaphroditism (compared to gonochorism) when reproductive contacts are frequent. However, even in widely dispersed or sluggish organisms behavioural mechanisms may exist which increase the frequency of reproductive contacts, favouring gonochorism instead of hermaphroditism.It is argued that externally fertilizing species should as a rule be gonochoric and that species which brood their young may often be hermaphroditic. Hermaphroditism in species which form permanent male/female pairs in the breeding season could result in more zygotes being produced. However, where parental care of the young is important, it is suggested that gonochorism and sexual dimorphism may result in more progeny being reared.     

Floral biology and pollination mechanisms of four Mexico-endemic Fuchsia species with contrasting reproductive systems

Aims The evolution of the outstanding variation of reproductive systems in angiosperms has been considered an important driver of lineage diversification. Closely related hermaphroditic and dioecious species with biotic pollination provide the opportunity to study and compare traits related to pollinator attraction and their consequences on reproductive components. A higher predictability of pollination syndromes is expected in dioecious species, which are dependent on pollinators, than in self-compatible hermaphroditic taxa. Dioecious species may suffer pollen limitation depending on the quality of floral rewards and the kind and abundance of pollinators, whereas no pollen limitation is expected in hermaphroditic species with autonomous self-pollination. Additionally, in the absence of pollen limitation, more or better seeds are expected in dioecious species, according to the sexual specialization hypothesis.Methods In natural populations of the hermaphroditic Fuchsia fulgens and Fuchsia arborescens and dioecious Fuchsia parviflora and Fuchsia obconica, all endemic to Mexico, we first described flower phenology, flower production and longevity and nectar volume and concentration. Then, we evaluated the correspondence between floral visitors and pollination syndromes. In hermaphrodite plants, we determined the level of herkogamy and the potential for autonomous self-pollination. Finally, we evaluated the effect of pollen limitation on fruit set and seed number and assessed seed germination for all species.Important findings In contrast to our prediction, dioecious species did not show a higher correspondence between pollination syndromes and floral visitors than did hermaphrodites; however, male flowers exhibited a higher correspondence than female flowers. No pollen limitation was detected in dioecious species, for which visitation rate did not differ between male and female flowers. The hermaphroditic F. fulgens showed pollen limitation for seed number, despite the presence of autonomous selfing. Fruit set from autonomous pollination was higher in F. arborescens, which showed a lower level of herkogamy compared with F. fulgens. Finally, dioecious species produced fewer but heavier seeds compared with hermaphrodite species. Although Fuchsia is classified as an outcrossing genus, both hermaphroditic species showed autonomous self-pollination. The heavier but lower number of seeds per fruit in dioecious species may be related to the more efficient resource allocation expected from sexual specialization. This could play an important role in the evolution of dioecy; however, a comparative phylogenetic approach is required to confirm this hypothesis.     

The unexpected mating system of the androdioecious barnacle Chelonibia testudinaria (Linnaeus 1758)

Androdioecy was first described by Darwin in his seminal work on barnacle diversity; he identified males and hermaphrodites in the same reproductive population. Today, we realize that many androdioecious plants and animals share astonishing similarities, particularly with regard to their evolutionary history and mating system. Notably, these species were ancestrally dioecious, and their mating system has the following characteristics: hermaphrodites self‐fertilize frequently, males are more successful in large mating groups, and males have a mating advantage. A male mating advantage makes androdioecy more likely to persist over evolutionary times. Androdioecious barnacles, however, appear to persist as an outlier with a different evolutionary trajectory: they originate from hermaphroditic species. Although sexual systems of androdioecious barnacles are known, no information on the mating system of androdioecious barnacles is available. This study assessed the mating system of the androdioecious barnacle Chelonibia testudinaria. In contrast to other androdioecious species, C. testudinaria does not self‐fertilize, males do not have a mating advantage over hermaphrodites, and the average mating group is quite small, averaging only three individuals. Mating success is increased by proximity to the mate and penis length. Taken together, the mating system of C. testudinaria is unusual in comparison with other androdioecious plants and animals, and the lack of a male mating advantage suggests that the mating system alone does not provide an explanation for the maintenance of androdioecy in this species. Instead, we propose that sex‐specific life history equalizes male and hermaphroditic overall fitness.     

Hermaphroditism in molluscs

The functional significance of hermaphroditism is, as yet, not well understood. This study attempts to examine the relevance of some theories on this topic to reality, by reviewing the literature on hermaphroditism among molluscs. The study reveals that about 40% of the 5600 mollusc genera are either simultaneous or sequential hermaphrodites. Hermaphroditism occurs in 100% of the Solenogastres, 99% of the Opisthobranchia, 100% of the Pulmonata, 3% of the Prosobranchia and 9% of the Bivalvia. Sequential hermaphroditism, whatever its theoretical advantages over gonochorism, is not common among molluscs. Simultaneous hermaphroditism is common among the Euthyneura. Over 80% of the hermaphroditic genera belong to the Euthyneura (about 2000 recent genera), and over 99% of the Euthyneura are hermaphrodites. The almost ubiquitous occurrence of hermaphroditism throughout the Euthyneurans, coupled with this group's very widespread dispersal in marine, terrestrial and freshwater habitats (and corresponding exposure to a very wide range of predators, competitors or parasites); and also its wide range of population densities, suggests that in this group hermaphroditism is not an adaptive response to selective forces of the environment and it may well be a phylogenetic constraint. A few stylommatophorans have broken this constraint by possessing genitalia in which the male system is absent (aphally) or extremely reduced (hemiphally). They reproduce by self-fertilization. In theory this is advantageous in that these species save the cost of developing two sets of reproductive apparatus. In reality there is no record of aphally occurring throughout an entire species, as one may expect if this saving of cost were so overwhelmingly advantageous. There is also no record of stylommatophoran individuals developing only the male system, as one may expect if there was any evolutionary trend from hermaphroditism towards gonochorism. Simultaneous hermaphroditism offers a (theoretical) option to selling. Selfing by self-fertilization is widespread among freshwater basommatophorans, amphibious stylommatophorans and freshwater bivalves. Selfing by parthenogenesis is widespread among freshwater prosobranchs. Altogether, of 48 genera which self, 60% are dwellers of freshwater. Certain freshwater habitats may perhaps be less stable than most marine or terrestrial ones are. Beyond the Euthyneura, hermaphroditism is common among parasitic, deep sea and Cnidaria-eating molluscs. Perhaps these three associations should be slightly rearranged and combined into a single generalization: that hermaphroditism (beyond the Euthyneura) is widespread among molluscs that live in a close and permanent intimacy with live marine invertebrates, whether as parasites, commensals or predators. Gonochorism but with dwarf males is common among the Eulimoidea, which are parasites, and Galeommatoidea, which are commensals. This situation of gonochorism, in which there is a very close association between members of the two sexes, is functionally a hermaphroditic type. Animals that can manipulate the physiology and endocrinology of their hosts may also have the tendency and ability to manipulate the size of their own males. It may well be that the female not only determines the size of the male but also his sex, in which case dwarf males should be considered as a case of environmental sex determination. The theoretical possibility that hermaphroditism may evolve in brooding animals was examined in gastropods. Among the (predominantly gonochoric) prosobranchs brooding is recorded in 85 genera (4%), only two of which are hermaphrodites. Among the (predominantly hermaphrodite) heterobranchs it is recorded in only 95 odd genera (4%). When all gastropod genera are considered together, the frequency of hermaphroditism among the brooders is only very slightly and insignificantly higher than in the non-brooders. When only the prosobranchs are considered, the frequency of hermaphroditism among the brooders is actually lower than among non-brooders. The theory that hermaphroditism evolves in brooding animals thus appears not to be relevant to the vast majority of gastropods. Brooding is widespread among freshwater prosobranchs (48% of the brooding genera). In this habitat, brooding may perhaps be an adaptation to protect the developing embryo against osmotic stresses.     

The transition to gender dimorphism on an evolutionary background of self-incompatibility: an example from Lycium (Solanaceae)

Populations of three North American species of Lycium (Solanaceae) are morphologically gynodioecious and consist of male-sterile (i.e., female) and hermaphroditic plants. Marked individuals were consistent in sexual expression across years and male sterility was present throughout much of the species' ranges. Controlled pollinations reveal that L. californicum, L. exsertum, and L. fremontii are functionally dioecious. Fruit production in females ranged from 36 to 63%, whereas hermaphrodites functioned essentially as males. Though hermaphrodites were mostly male, investigation of pollen tube growth reveals that hermaphrodites of all dimorphic species were self-compatible. Self-fertilization and consequent inbreeding depression are commonly invoked as important selective forces promoting the invasion of male-sterile mutants into cosexual populations. A corollary prediction of these models is that gender dimorphism evolves from self-compatible ancestors. However, fruit production, seed production, and pollen tube number following outcross pollination were significantly higher than following self-pollination for three diploid, cosexual species that are closely related to the dimorphic species. The data presented here on incompatibility systems are consistent with the hypothesis that polyploidy disrupted the self-incompatibility system in the gynodioecious species leading to the evolution of gender dimorphism.     

Sexual system in the tanaidacean Falsapseudes bowmani (Crustacea: Malacostraca: Peracarida)

With more than 40,000 species, Malacostraca is the most diverse crustacean class. Most malacostracans are gonochoristic, but simultaneous hermaphrodites are also known. Tanaidacea is one of two malacostracan orders that includes simultaneously hermaphroditic species; so far, simultaneous hermaphroditism has been confirmed externally and internally in only two tanaidacean species, both in the genus Apseudes (Apseudidae). Here we show, through external and internal morphological observations of fixed specimens, that the apseudid Falsapseudes bowmani is a simultaneous hermaphrodite, making Falsapseudes the second tanaidacean genus in which simultaneous hermaphroditism has been confirmed both externally and internally. In this species, the epistome (a projection on the clypeus) was thick and elongate in large specimens but was thin and spiniform in smaller specimens; the brooding of eggs or embryos was observed only in thin‐epistome individuals, although a pair of ovaries was confirmed in both thick‐ and thin‐epistome individuals. This suggests that individuals with a thick epistome may act as males while also retaining the female reproductive organs.     

Variation in reproductive success and gonadal allocation in the simultaneous hermaphrodite,Serranus fasciatus

Summary This paper examines the correlates of individual size, reproductive success, gonadal allocation, and growth in a hermaphroditic reef fish. Individuals in S. fasciatus mature as simultaneous hermaphrodites; large individuals subsequently lose female function and become functional males. Daily female reproductive success was highly correlated with both hermaphrodite size and amount of female gonadal tissue. Three separate comparisons gave a positive correlation between male reproductive success and male gonadal allocation: (1) Males had higher levels of male gonadal allocation and male reproductive success than hermaphrodites. (2) The percent of gonad allocated to male tissue in hermaphrodites was higher in the year they had higher male mating success. (3) Male gonadal tissue of hermaphrodites was positively correlated with male reproductive success in the year that male reproductive success by hermaphrodites was higher and more variable. There was no evidence for a trade-off between male function, female function, and growth among hermaphrodites. Many of these patterns have also been observed in plants, but the selective pressures leading to these patterns in S. fasciatus and plants are probably quite different.