Sequential hermaphroditism (called dichogamy in botany) is a type of hermaphroditism that occurs in many fish, gastropods, and plants. Sequential hermaphroditism occurs when the individual changes sex at some point in its life.
In animals, the different types of change are male to female (protandry), female to male (protogyny), female to hermaphrodite (protogynous hermaphroditism), and male to hermaphrodite (protandrous hermaphroditism). Those that change gonadal sex can have both female and male germ cells in the gonads or can change from one complete gonadal type to the other during their last life stage.
In plants, individual flowers are called dichogamous if their function has the two sexes separated in time, although the plant as a whole may have functionally male and functionally female flowers open at any one moment. A flower is protogynous if its function is first female, then male, and protandrous if its function is male then female. It used to be thought that this reduced inbreeding, but it may be a more general mechanism for reducing pollen-pistil interference.
Protandrous hermaphrodites are animals that are born male and at some point in their lifespan change sex to female. Protandrous animals include clownfish. Clownfish have a very structured society. In the Amphiprion percula species, there are zero to four individuals excluded from breeding and a breeding pair living in a sea anemone. Dominance is based on size, the female being the largest and the male being the second largest. The rest of the group is made up of progressively smaller non-breeders, which have no functioning gonads. If the female dies, the male gains weight and becomes the female for that group. The largest non-breeding fish then sexually matures and becomes the male of the group.
Other protandrous fishes can be found in the classes clupeiformes, siluriformes, stomiiformes, and within the perciform families pomacentridae and sparidae. Since these groups are distantly related and have many intermediate relatives that are not protandrous, it strongly suggests that protandry evolved multiple times. Other examples of protandrous animals include:
- The ctenophore Coeloplana gonoctena. In this organism the females are bigger than the males and are only found during the summer. In contrast males are found year-round.
- The flatworms Hymanella retenuova and Paravortex cardii.
- Laevapex fuscus, a gastropod, is described as being functionally protandric. The sperm matures in late winter and early spring, the eggs mature in early summer, and copulation occurs only in June. This shows that males cannot reproduce until the females appear, thus why they are considered to be functionally protandric.
- Speyeria mormonia, or the Mormon Fritillary, is a butterfly species exhibiting protandry. In its case, functional protandry refers to the emergence of male adults 2–3 weeks before female adults.
Protogynous hermaphrodites are animals that are born female and at some point in their lifespan change sex to male. As the animal ages, based on internal or external triggers, it shifts sex to become a male animal. Male fecundity increases greatly with age, unlike female.
Protogyny is the most common form of hermaphroditism in fish in nature. About 75% of the 500 known sequentially hermaphroditic fish species are protogynous. Common model organisms for this type of sequential hermaphroditism are wrasses. They are one of the largest families of coral reef fish and belong to the family Labridae. Wrasses are found around the world in all marine habitats and tend to bury themselves in sand at night or when they feel threatened. In wrasses, the larger of the two fish is the male, while the smaller is the female. In most cases, females and immature have a uniform color while the male has the terminal bicolored phase. Large males hold territories and try to pair spawn while small to mid-size initial-phase males live with females and group spawn. In other words, both the initial and terminal phase males can breed, but they differ in the way they do it.
In the California sheephead (Semicossyphus pulcher), a type of wrasse, when the female changes to male, the ovaries degenerate and spermatogenic crypts appear in the gonads. The general structure of the gonads remains ovarian after the transformation and the sperm is transported through a series of ducts on the periphery of the gonad and oviduct. Here sex change is age-dependent. For example, the California sheephead stays a female for four years before changing sex.
Blue-headed wrasse begin life as males or females, but females can change sex and function as males. Young females and males start with a distinct coloration known as the "Initial Phase" before progressing into the "Terminal Phase" coloration, which has a change in intensity of color, stripes, and bars. Initial Phase males have larger testes than larger, terminal phase males, which enables the initial phase males to produce a large amount of sperm. This strategy is able to compete with that of the larger male, who is able to guard his own harem.
Botryllus schlosseri, a colonial tunicate, is a protogynous hermaphrodite. In a colony, eggs are released about two days before the peak of sperm emission. Although self-fertilization is avoided and cross-fertilization favored by this strategy, self-fertilization is still possible. Self-fertilized eggs develop with a substantially higher frequency of anomalies during cleavage than cross-fertilized eggs (23% vs. 1.6%). Also a significantly lower percentage of larvae derived from self-fertilized eggs metamorphose, and the growth of the colonies derived from their metamorphosis is significantly lower. These findings suggest that self-fertilization gives rise to inbreeding depression associated with developmental deficits that are likely caused by expression of deleterious recessive mutations.
Other examples of protogynous organisms include:
- In the following fish families: Serranidae (groupers), Sparidae (porgies), Synbranchidae (swamp eels), Labridae (wrasses), Scaridae (parrotfishes), Pomacanthidae (angelfishes), Gobiidae (gobies), Elacatinus, Lethrinidae (emperors), and possibly others.
- The isopods Cyathura polita and C. carinata
- The tanaidacean Heterotanais oerstedi.
- The echinoderms, Asterina pancerii and A. gibbosa are also protogynous and they brood their young.
- Protogyny sometimes occurs in the frog Rana temporaria, where old females sometimes change to males.
The ultimate cause of a biological phenomenon concerns why that phenomenon makes organisms better adapted to their environment, and thus why evolution by natural selection has produced that phenomenon. This is in contrast to proximate causes, which concern the molecular and physiological mechanisms that produce the phenomenon. A number of ultimate causes of hermaphroditism have been proposed, of which two are most relevant to sequential hermaphroditism.
The 'size-advantage model' states that individuals of a given sex reproduce more effectively if they are a certain size or age. To create selection for sequential hermaphroditism, small individuals must have higher reproductive fitness as one sex and larger individuals must have higher reproductive fitness as the opposite sex. For example, eggs are larger than sperm, thus larger individuals are able to make more eggs, so individuals could maximize their reproductive potential by beginning life as male and then turning female upon achieving a certain size.
Sequential hermaphroditism can also protect against inbreeding in populations of organisms that have low enough motility and/or are sparsely distributed enough that there is a considerable risk of siblings encountering each other after reaching sexual maturity, and interbreeding. If siblings are all the same or similar ages, and if they all begin life as one sex and then transition to the other sex at about the same age, then siblings are highly likely to be the same sex at any given time. This should dramatically reduce the likelihood of inbreeding. Both protandry and protogyny are known to help prevent inbreeding in plants, and many examples of sequential hermaphroditism attributable to inbreeding prevention have been identified in a wide variety of animals.
In most ectotherms body size and female fecundity are positively correlated. This supports the size-advantage model. Kazancioglu and Alonzo (2010) performed the first comparative analysis of sex change in Labridae. Their analysis supports the size-advantage model and suggest that sequential hermaphroditism is correlated to the size-advantage. They determined that dioecy was less likely to occur when the size advantage is stronger than other advantages Warner suggests that selection for protandry may occur in populations where female fecundity is augmented with age and individuals mate randomly. Selection for protogyny may occur where there are traits in the population that depress male fecundity at early ages (territoriality, mate selection or inexperience) and when female fecundity is decreased with age, the latter seems to be rare in the field. An example of territoriality favoring protogyny occurs when there is a need to protect their habitat and being a large male is advantageous for this purpose. In the mating aspect, a large male has a higher chance of mating, while this has no effect on the female mating fitness. Thus, he suggests that female fecundity has more impact on sequential hermaphroditism than the age structures of the population.
The size-advantage model predicts that sex change would only be absent if the relationship between size/age with reproductive potential is identical in both sexes. With this prediction one would assume that hermaphroditism is very common, but this is not the case. Sequential hermaphroditism is very rare and according to scientists this is due to some cost that decreases fitness in sex changers as opposed to those who don't change sex. Some of the hypotheses proposed for the dearth of hermaphrodites are the energetic cost of sex change, genetic and/or physiological barriers to sex change, and sex-specific mortality rates.
In 2009, Kazanciglu and Alonzo found that dioecy was only favored when the cost of changing sex was very large. This indicates that the cost of sex change does not explain the rarity of sequential hermaphroditism by itself.
Many studies have focused on the proximate causes of sequential hermaphroditism. The role of aromatase has been widely studied in this area. Aromatase is an enzyme that controls the androgen/estrogen ratio in animals by catalyzing the conversion of testosterone into oestradiol, which is irreversible. It has been discovered that the aromatase pathway mediates sex change in both directions. Many studies also involve understanding the effect of aromatase inhibitors on sex change. One such study was performed by Kobayashi et al. In their study they tested the role of estrogens in male three-spot wrasses (Halichoeres trimaculatus). They discovered that fish treated with aromatase inhibitors showed decreased gonodal weight, plasma estrogen level and spermatogonial proliferation in the testis as well as increased androgen levels. Their results suggest that estrogens are important in the regulation of spermatogenesis in this protogynous hermaphrodite.
Sequential hermaphrodites almost always have a sex ratio biased towards the birth sex, and consequently experience significantly more reproductive success after switching sexes. In theory, this should decrease genetic diversity and effective population size (Ne). However, a study of two ecologically similar santer sea bream (gonochoric) and slinger sea bream (protogynous) in South African waters found that genetic diversities were similar in the two species, and while Ne was lower in the instant for the sex-changer, they were similar over a relatively short time horizion.
In the context of the plant sexuality of flowering plants (angiosperms), there are two forms of dichogamy: protogyny—female function precedes male function—and protandry—male function precedes female function.
Historically, dichogamy has been regarded as a mechanism for reducing inbreeding. However, a survey of the angiosperms found that self-incompatible (SI) plants, which are incapable of inbreeding, were as likely to be dichogamous as were self-compatible (SC) plants. This finding led to a reinterpretation of dichogamy as a more general mechanism for reducing the impact of pollen-pistil interference on pollen import and export. Unlike the inbreeding avoidance hypothesis, which focused on female function, this interference-avoidance hypothesis considers both reproductive functions.
In many hermaphroditic species, the close physical proximity of anthers and stigma makes interference unavoidable, either within a flower or between flowers on an inflorescence. Within-flower interference, which occurs when either the pistil interrupts pollen removal or the anthers prevent pollen deposition, can result in autonomous or facilitated self-pollination. Between-flower interference results from similar mechanisms, except that the interfering structures occur on different flowers within the same inflorescence and it requires pollinator activity. This results in geitonogamous pollination, the transfer of pollen between flowers of the same individual. In contrast to within-flower interference, geitonogamy necessarily involves the same processes as outcrossing: pollinator attraction, reward provisioning, and pollen removal. Therefore, between-flower interference not only carries the cost of self-fertilization (inbreeding depression), but also reduces the amount of pollen available for export (so-called "pollen discounting"). Because pollen discounting diminishes outcross siring success, interference avoidance may be an important evolutionary force in floral biology. Dichogamy may reduce between-flower interference by minimizing the temporal overlap between stigma and anthers within an inflorescence. Large inflorescences attract more pollinators, potentially enhancing reproductive success by increasing pollen import and export. However, large inflorescences also increase the opportunities for both geitonogamy and pollen discounting, so that the opportunity for between-flower interference increases with inflorescence size. Consequently, the evolution of floral display size may represent a compromise between maximizing pollinator visitation and minimizing geitonogamy and pollen discounting (Barrett et al., 1994).
Protandry may be particularly relevant to this compromise, because it often results in an inflorescence structure with female phase flowers positioned below male phase flowers. Given the tendency of many insect pollinators to forage upwards through inflorescences, protandry may enhance pollen export by reducing between-flower interference. Furthermore, this enhanced pollen export should increase as floral display size increases, because between-flower interference should increase with floral display size. These effects of protandry on between-flower interference may decouple the benefits of large inflorescences from the consequences of geitonogamy and pollen discounting. Such a decoupling would provide a significant reproductive advantage through increased pollinator visitation and siring success.
Harder et al. (2000) demonstrated experimentally that dichogamy both reduced rates of self-fertilization and enhanced outcross siring success through reductions in geitonogamy and pollen discounting, respectively. Routley & Husband (2003) examined the influence of inflorescence size on this siring advantage and found a bimodal distribution with increased siring success with both small and large display sizes.
The length of stigmatic receptivity plays a key role in regulating the isolation of the male and female stages in dichogamous plants, and stigmatic receptivity can be influenced by both temperature and humidity. Another study by Jersakova and Johnson, studied the effects of protandry on the pollination process of the moth pollinated orchid, Satyrium longicauda. They discovered that protandry tended to reduce the absolute levels of self-pollination and suggest that the evolution of protandry could be driven by the consequences of the pollination process for male mating success. Another study that indicated that dichogamy might increase male pollination success was by Dai and Galloway.
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