Sex determination in the androdioecious barnacle Scalpellum scalpellum (Crustacea: Cirripedia)

How androdioecy (coexistence of hermaphrodites and males) is maintained is still poorly understood. Therefore, sex determination was studied in the androdioecious barnacle Scalpellum scalpellum L. First, 247 cypris larvae from seven broods were investigated for sexual dimorphism in larval morphology and found to be all identical. Second, experiments with cyprids showed that males and hermaphrodites differ distinctly in morphology as soon as 4–5 days after settlement. Third, 14 252 cyprids were allowed to settle on the bottom of their culture cages, and all surviving larvae developed into hermaphrodites and none into dwarf males. Fourth, larvae settled in hermaphrodite receptacles (i.e. future males) were removed at increasing intervals after settlement to study if the male and hermaphrodite sexual expressions are fixed or plastic. All larvae became dwarf males if allowed to stay there for more than 8 h after settlement. But if removed within 3 h after settlement, half of them developed into hermaphrodites. We conclude that an environmental sex determination mechanism operates in S. scalpellum. Together with a 1:1 hermaphrodite/male ratio observed in previously reported experiments offering a free choice of settlement, we suggest that all larvae are potential hermaphrodites, but only 50% can settle in hermaphrodite receptacles and yield males.     

Reproductive strategy of two deep-sea scalpellid barnacles (Crustacea: Cirripedia: Thoracica) associated with decapods and pycnogonids and the first description of a penis in scalpellid dwarf males

We investigated the sexual system in two pedunculate barnacles of the family Scalpellidae. Both inhabit deep water and are attached to mobile arthropod hosts. Verum brachiumcancri was attached to the majid crab Rochinia hertwigi, and Weltnerium nymphocola to the pycnogonidan sea spider Boreonymphon rubrum. Both barnacles have separate sexes and females almost always carry two dwarf males that are almost fully embedded in a pair of symmetrically situated receptacles inside the rim of the mantle cavity. The dwarf males of V. brachiumcancri have a complex penile structure extending into the female mantle cavity. This is the first time a copulatory structure has been described in detail for a dwarf male of a scalpellid barnacle. Both species lack free nauplii and their larvae are released as cyprids; the brood size is small. This is probably an adaptation for settling close to the parent population. We compare reproductive strategies among scalpellids and suggest that the present males are highly specialized and that the females are allocating resources to few offspring.     

Male–Male Competition in a Mixed‐Mating Fish

Mating systems that comprise a mixture of pure males and self‐fertilising hermaphrodites remain an evolutionary enigma. In particular, our understanding of the sexual selection pressures associated with such mating systems is nascent. Males can only reproduce by fertilising hermaphrodites’ eggs, but hermaphrodites can also fertilise their own eggs and gain a genetic advantage by doing so. Consequently, there should be intense competition among males to access hermaphrodites. Here, we test the importance of male size, colour and heterozygosity in predicting the outcome of male–male competition using the mangrove rivulus, which has a male‐hermaphrodite mixed‐mating system. We pitted males against one another in dyadic laboratory trials to develop a dominance score for each male. We then correlated these scores with male length, several components of male colour, and heterozygosity. Male size was the only significant correlate of dominance: larger males dominated smaller males, implying selection for large male size. However, male mangrove rivulus are similar in size to hermaphrodites, indicating that directional selection for large body size in males is no greater than it is in hermaphrodites. Across all trials, colour was unrelated to dominance, but contests between similarly sized males were usually won by more colourful individuals. As mangrove rivulus are dichromatic, we suspect that male colour may prove to be more important in mate choice than we found it to be in intrasexual competition. Heterozygosity did not explain dominance directly, but correlated strongly with male size, implying an indirect role in intrasexual competition.     

Reproduction in the genus Limacina (Opisthobranchia: Thecosomata)

Reproductive mechanisms in the seven species of the thecosomatous pteropod genus Limacina are described and compared. All species are protandrous hermaphrodites. Five species– L. bulimoides, L. helicina, L. lesueuri, L. retroversa and L. trochiformis –have a similar reproductive anatomy in which the gonoduct leading from the gonad to the common genital pore functions as a seminal vesicle in the male and is elaborated into mucous and albumen glands in the female. The male system consists of a prostate gland and penis connected to the common genital pore by an external ciliary tract. All five species have a free-swimming veliger stage which hatches from free-floating egg masses. Limacina helicoides has the same reproductive anatomy but is ovoviviparous, with embryos retained in capsules in the mucous gland until they are juveniles of 50 mm in shell diameter. Limacina inflata lacks mucous and albumen glands and a penis; a spermatophore formed by the prostate gland is used in aphallic sperm transfer. This species exhibits brood protection with un-encapsulated embryos retained in the mantle cavity until they are released as veligers measuring 0067 mm in diameter. L. inflata is the most abundant of the seven species despite lowered fecundity; reasons for its ecological success are discussed.     

Sperm status regulates sexual attraction in Caenorhabditis elegans

Mating behavior of animals is regulated by the sensory stimuli provided by the other sex. Sexually receptive females emit mating signals that can be inhibited by male ejaculate. The genetic mechanisms controlling the release of mating signals and encoding behavioral responses remain enigmatic. Here we present evidence of a Caenorhabditis elegans hermaphrodite-derived cue that stimulates male mating-response behavior and is dynamically regulated by her reproductive status. Wild-type males preferentially mated with older hermaphrodites. Increased sex appeal of older hermaphrodites was potent enough to stimulate robust response from mating-deficient pkd-2 and lov-1 polycystin mutant males. This enhanced response of pkd-2 males toward older hermaphrodites was independent of short-chain ascaroside pheromones, but was contingent on the absence of active sperm in the hermaphrodites. The improved pkd-2 male response toward spermless hermaphrodites was blocked by prior insemination or by genetic ablation of the ceh-18-dependent sperm-sensing pathway of the hermaphrodite somatic gonad. Our work suggests an interaction between sperm and the soma that has a negative but reversible effect on a hermaphrodite-derived mating cue that regulates male mating response, a phenomenon to date attributed to gonochoristic species only.     

Implications for the Maintenance of Androdioecy in the Freshwater Shrimp, Eulimnadia texana Packard: Encounters between Males and Hermaphrodites are not Random

Androdioecy is a mixed‐mating system in which there are males and hermaphrodites but no pure females. Few species exhibit such a mating system. Eulimnadia texana is a branchiopod crustacean that has recently been identified as an androdioecious species. This system is ideal for testing questions related to the evolution of sexual reproduction. We are testing a model that predicts androdioecy to be a stable mixed‐mating system under certain conditions. Specifically, we investigated whether encounters between males and hermaphrodites are random or if either sex seeks out the other for mating. Focal male or hermaphrodite clam shrimp were presented with stimulus shrimp of the other sex or kept alone. Swimming speed and time spent within different areas of a test chamber were recorded. Males did not alter mean swimming speed or spend more time than expected by chance near partitioned hermaphrodites. Hermaphrodites, however, decreased mean swimming speed in the presence of males and also spent more time than expected by chance near partitioned males, suggesting that hermaphrodites respond to male chemical and/or visual stimuli. Modified swimming behaviour probably facilitates inter‐sexual contact, thereby increasing opportunities for out‐crossing above that expected by random encounters.     

Brooding and the evolution of hermaphroditism

It has been suggested that hermaphroditism may evolve when the resources that females can profitably allocate to ova is limited by factors such as lack of brooding space. Spare resources could then be allocated to produce male gametes in a hermaphrodite. A model is developed to investigate the conditions under which this will occur. Hermaphrodites will displace males (and females) if the hermaphrodites produce at least half as many male gametes as a male. If the hermaphrodite produces less than half the number of gametes produced by a male then a stable equilibrium arises where males and hermaphrodites coexist. In this situation the frequency of males is determined by the ratio of the numbers of male gametes produced by hermaphrodites to the numbers produced by males.     

Do simultaneous hermaphrodites choose their mates? Effects of body size in a planarian flatworm

1. We tested whether or not the simultaneous hermaphrodite Dugesia polychroa mates randomly with respect to partner size, a strong indicator of female fecundity.
2. The number and duration of copulations were recorded in forty-eight pairs over 5 days. To minimize effects of individual differences in copulatory activity, all animals were each tested twice: once with an equally large partner ( n = 24 equal-sized pairs) and once with a partner of different size (smaller or larger, respectively, ratio 1 : 1.5,
n = 24 unequal-sized pairs).
3. The animals copulated frequently. No significant differences were observed between treatments in the number of pairs that copulated. Mating behaviour of equal-sized and unequal-sized pairs was compared in a pair-wise analysis. Latency to the first copulation and the average number of copulations did not differ when mating with an equal- or unequal-sized partner. Average copulation duration increased with increasing size in equal-sized pairs, but not in unequal-sized pairs.
4. In an additional experiment, where three large and three small animals were combined (three replicates) and observed for 1–3 days, the mating pattern did not differ from random mating by size.
5. Our data indicate that in D. polychroa the decision to mate is essentially random with respect to size. We argue that this is not caused by, for example, high costs of mate searching, but that simultaneous hermaphrodites with reciprocal insemination mate indiscriminately when (i) matings are easy to obtain and (ii) the cost of mating is low or compensated for by sperm digestion.     

Do simultaneous hermaphrodites choose their mates? Effects of body size in a planarian flatworm

1. We tested whether or not the simultaneous hermaphrodite Dugesia polychroa mates randomly with respect to partner size, a strong indicator of female fecundity.
2. The number and duration of copulations were recorded in forty-eight pairs over 5 days. To minimize effects of individual differences in copulatory activity, all animals were each tested twice: once with an equally large partner ( n = 24 equal-sized pairs) and once with a partner of different size (smaller or larger, respectively, ratio 1 : 1.5,
n = 24 unequal-sized pairs).
3. The animals copulated frequently. No significant differences were observed between treatments in the number of pairs that copulated. Mating behaviour of equal-sized and unequal-sized pairs was compared in a pair-wise analysis. Latency to the first copulation and the average number of copulations did not differ when mating with an equal- or unequal-sized partner. Average copulation duration increased with increasing size in equal-sized pairs, but not in unequal-sized pairs.
4. In an additional experiment, where three large and three small animals were combined (three replicates) and observed for 1–3 days, the mating pattern did not differ from random mating by size.
5. Our data indicate that in D. polychroa the decision to mate is essentially random with respect to size. We argue that this is not caused by, for example, high costs of mate searching, but that simultaneous hermaphrodites with reciprocal insemination mate indiscriminately when (i) matings are easy to obtain and (ii) the cost of mating is low or compensated for by sperm digestion.     

Males outcompete hermaphrodites for seed siring success in controlled crosses in the polygamous Fraxinus excelsior (Oleaceae)

Polygamy (including trioecy and subdioecy), the co-occurrence of males, hermaphrodites, and females in natural populations, is a rare and poorly studied breeding system expressed in Fraxinus excelsior L. (Oleaceae), a wind-pollinated tree. Here we investigate siring ability of pollen from male vs. hermaphrodite individuals to better understand this sex polymorphism. We conducted single-donor and two-donor pollination experiments and compared both fruit set and seed siring success, assessed with polymorphic microsatellite markers, of male and hermaphrodite individuals. Single pollen donor crosses allowed us to verify the male function of hermaphrodites. However, pollen from hermaphrodites was much less proficient than male pollen, with males siring 10 times as many fruits in single donor pollination treatments. This result was strengthened by the surprisingly low reproductive success of hermaphrodites in pollen competition conditions: of the 110 seedlings analyzed three were selfed and only one was sired by the hermaphrodite donor. The remaining 106 were sired by the male pollen donor. These results raise the question of the maintenance of male fertility in hermaphrodites in Fraxinus excelsior. Male function of hermaphrodites in this species now needs to be assessed under field conditions.     

Mating system and protandrous sex change in the lizard flathead, <Emphasis Type="Italic">Inegocia japonica</Emphasis> (Platycephalidae)

Mating system and protandrous sex change of the lizard flathead Inegocia japonica were investigated off Nagashima, Kagoshima, Japan. The reproductive season of this species was estimated to be from June to September by a fluctuation of gonadsomatic index from monthly sampling data. The hermaphrodite fish, having testicular and ovarian parts, were collected from February to December, indicating that the sex change occurred in both reproductive and nonreproductive seasons. Females were larger than males and hermaphrodites. Moreover, five cases of the sex change from male to female were directly confirmed during the 1995–1997 field census in the behavioral observation area. Home ranges, where no aggressive interaction took place among any individuals, overlapped within and between sexes. Seven pair spawnings observed in the area were not of the same individual combination, and the size of pair fish was likely to be random. This species may thus have a promiscuous mating system, i.e., random pairing, which favors protandry as predicted in the size-advantage model.     

Accessory male investment can undermine the evolutionary stability of simultaneous hermaphroditism

Sex allocation (SA) models are traditionally based on the implicit assumption that hermaphroditism must meet criteria that make it stable against transition to dioecy. This, however, puts serious constraints on the adaptive values that SA can attain. A transition to gonochorism may, however, be impossible in many systems and therefore realized SA in hermaphrodites may not be limited by conditions that guarantee stability against dioecy. We here relax these conditions and explore how sexual selection on male accessory investments (e.g. a penis) that offer a paternity benefit affects the evolutionary stable strategy SA in outcrossing, simultaneous hermaphrodites. Across much of the parameter space, our model predicts male allocations well above 50 per cent. These predictions can help to explain apparently ‘maladaptive’ hermaphrodite systems.     

Polygynous Mating System of Trimma okinawae (Pisces: Gobiidae) at Kagoshima,Japan with a Note on Sex Change

Mating system, social structure and reproductive behaviour of the gobiid fish Trimma okinawae were studied at Akamizu, Kagoshima, Japan. Mating system of this species is polygyny. There were two types of habitat-related distribution patterns of male and female home ranges: females have home ranges outside a small territory of the male; female home ranges are covered by male territory. Spawnings take place with a cycle of four to five days during the spawning season of June to Sept. In the morning of the day of spawning, males stay at the spawning sites. Females visit the site and spawn with the male. On days between spawnings males exhibit courtship displays at female home ranges. Higher reproductive success is not connected with frequency of such courting of the male, but with male size. The largest female of a social unit changes into a male when the dominant male is removed, which shows that T. okinawae is a hermaphrodite.     

Self-Incompatibility and Male Fertilization Success in Phillyrea angustifolia (Oleaceae)

Androdioecy is a rare breeding system in which low male frequency is expected in populations because males require a strong increase in their fertility to be maintained by selection. Phillyrea angustifolia L. has previously been reported as possibly functionally androdioecious. However, 1&rcolon;1 sex ratios have been reported and suggest functional dioecy. In this article, we compared both pollen tube growth and siring success of male and hermaphrodite pollen in two single-donor pollination experiments. We verified at both pre- and postzygotic levels that hermaphrodites produce functional pollen. Self-incompatibility was also clearly established. However, pollen from hermaphrodites was less efficient than male pollen. The probability of a pollen tube growing through the style was higher for male than for hermaphrodite pollen donors, and males sired twice as many fruits as hermaphrodites. The twofold male advantage in relative fecundity was mainly because of lower pollen fertility of hermaphrodites and possible cross-incompatibility among hermaphrodites.     

Sex‐specific reproductive components and pollination ecology in the subdioecious shrub Fuchsia microphylla

In subdioecious populations, functional female, male and hermaphrodite individuals coexist. Subdioecy may be a transitional state towards dioecy or a breakdown of dioecy, although lability in sex expression may maintain subdioecy as a stable condition. To better understand the ecological aspects involved in sex ratio dynamics and breeding system evolution, we studied the pollination and female fitness components of female and hermaphrodite individuals of the subdioecious shrub Fuchsia microphylla. In two natural populations at the Trans‐Mexican Volcanic Belt we estimated female frequency and several reproductive components of female and hermaphrodite plants under natural pollination and experimental pollination treatments. Average female frequency was 42%, and on average, 42.5% of hermaphrodites produced fruits. Female plants showed a 17‐fold female fertility advantage over hermaphrodites through increased fruit production, as the number of seeds and germination rates did not differ between morphs. Hermaphrodite flowers were larger, with similar nectar production and concentration to female flowers, and pollinators did not show consistent morph preferences. Some hermaphrodites produced fruits autonomously, and female flowers excluded from pollinators produced fruits putatively by apomixis. Fruit production in hermaphrodites, but not in females, was related to height, suggesting increased investment of hermaphrodites in the female function at higher resource status. For sex ratios to be at equilibrium, the female fertility advantage should be reduced about eightfold. However, it may be that hermaphrodites are maintained by producing fruits at no cost to the male function at higher resource status, as the gender plasticity hypothesis proposes.     

Mate choice in a hermaphrodite: you won't score with a spermatophore

Sexual selection has played a major role in shaping the wide variety of mating patterns found in species with separate sexes, but little is known about its effects on simultaneous hermaphrodites. However, many hermaphrodites possess complex reproductive systems and mating behaviour is often elaborate, suggesting that some form of mate assessment takes place. We found that the marine slug Aeolidiella glauca, a simultaneous hermaphrodite with reciprocal external sperm transfer via spermatophores, shows a unique mate choice behaviour by avoiding mating with conspecifics already carrying a spermatophore received during the previous mating. Current mating status did not seem to affect this behaviour, because both slugs that had mated 2-3 days before our mate choice trials and slugs that had been isolated for 4-6 weeks avoided spermatophore-carrying partners. There are two obvious reasons why slugs should avoid recently mated partners. First, they may reduce the risk of getting a partner depleted in self-sperm. Second, the risk of sperm competition may be decreased. Histological investigations of sperm reserves suggest that sperm depletion did not influence our mate choice experiments. Most slugs had sufficient sperm stored for spermatophore production. Therefore, the most likely explanation for A. glauca's peculiar mate choice is that, by avoiding a recently mated partner, a sperm donor may reduce its risk of being subjected to sperm competition.     

SEXUAL ALLOCATION AND ANDROMONOECY: RESOURCE INVESTMENT IN MALE AND HERMAPHRODITE FLOWERS OF SOLANUM CAROLINENSE (SOLANACEAE)

Male and hermaphrodite flowers from the andromonoecious perennial Solanum carolinense were compared. Hermaphrodite flowers were 11–16% heavier than males, and this difference was consistent for two clones studied. Hermaphrodite flowers contained a greater amount of nitrogen than did male flowers, but males had higher percent nitrogen. Male and hermaphrodite flowers had equal investment in male structures: stamen mass, number of pollen grains, mass of pollen grains ejected by vibration, and pollen size were all similar for the two flower types. In contrast, male flowers had reduced masses of disfunctional female structures: ovaries of males were ⅓ as massive, and styles 1/7; as massive, as those of hermaphrodites. These results are consistent with the hypothesis that male flowers (which occur where fruit set is unlikely) increase male fitness through pollen donation, while saving resources that would otherwise be invested in hermaphrodite flowers that abscise.     

Evolution of increased self-sperm production in postdauer hermaphroditic nematodes

A recent study suggests that postdauer Caenorhabditis elegans hermaphrodites produce more self‐sperm and have larger brood sizes than worms that bypass diapause. Why might natural selection favor increased self‐sperm production in postdauer hermaphrodites? This question is addressed by developing an age‐structured model for an exponentially growing worm population descending from a founder postdauer hermaphrodite. It is assumed that natural selection favors those founders that have the largest number of living descendants at some fixed future time. Increased self‐sperm production in postdauer hermaphrodites can then evolve when the diapause‐bypassing descendants suffer a higher mortality rate than their parental postdauer founders.     

Phenotypically plastic adjustment of sex allocation in a simultaneous hermaphrodite

Sex allocation theory for simultaneous hermaphrodites predicts an influence of the mating group size on sex allocation. Mating group size may depend on the size of the group in which an individual lives, or on the density, but studies to date have not distinguished between the two factors. We performed an experiment in which we raised a transparent simultaneous hermaphrodite, the flatworm Macrostomum sp., in different group sizes (pairs, triplets, quartets and octets) and in different enclosure sizes (small and large). This design allows us to differentiate between the effects of group size and density. After worms reached maturity we determined their reproductive allocation patterns from microscopic images taken in vivo. The results suggest that the mating group size is a function of the group size, and not of the density. They support the shift to higher male allocation in larger mating groups predicted by sex allocation theory. To our knowledge, this is the first study that unambiguously shows phenotypically plastic sex allocation in response to mating group size in a simultaneous hermaphrodite.