Sequential hermaphroditism
Species that can undergo these changes do so as a normal event within their reproductive cycle, usually cued by either social structure or the achievement of a certain age or size.
[5] These various types of sequential hermaphroditism may indicate that there is no advantage based on the original sex of an individual organism.
[11] However, protandry features a spectrum of different forms, which are characterized by the overlap between male and female reproductive function throughout an organism's lifetime: Furthermore, there are also species that reproduce as both sexes throughout their lifespans (i.e simultaneous hermaphrodites), but shift their reproductive resources from male to female over time.
[14] In fact, protandrous hermaphroditism occurs in many fish,[15] mollusks,[12] and crustaceans,[16] but is completely absent in terrestrial vertebrates.
[29] In many fishes, female fecundity increases continuously with age, while in other species larger males have a selective advantage (such as in harems), so it is hypothesized that the mating system can determine whether it is more selectively advantageous to be a male or female when an organism's body is larger.
This causes small males to have a severe reproductive disadvantage, which promotes strong selection of size-based protogyny.
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 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.
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.
[55] Warner suggests that selection for protandry may occur in populations where female fecundity is augmented with age and individuals mate randomly.
[4] 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.
Therefore, in this kind of paired mating system, protandry is the most adaptive strategy ("breed as a male when small, and then change to female when you're larger").
[citation needed] 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.
During sex reversal, their whole gonads including the germinal epithelium undergoes significant changes, remodeling, and reformation.
Thus, the synthesis of sex steroids coincides with gonadal remodeling, which is triggered by MMPs produced by germinal epithelial tissue.
These results suggests that MMPs and changes in steroid levels play a large role in sequential hermaphroditism in teleosts.
[65] The Patchy Environment Model and Size Dependent Sex Allocation are the two environmental factors which drive sequential hermaphroditism in plants.
Evolutionarily, sequential hermaphrodites emerged as certain species obtained a reproductive advantage by changing their sex.
[citation needed] Arisaema triphyllum (Jack in the pulpit) is a plant species which is commonly cited as exercising sequential hermaphroditism.
[67] Striped maple trees (Acer pensylvanicum) have been shown to change sex over a period of several years, and are sequential hermaphrodites.
[71] 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.
[73][9] 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.
[74][73] In contrast to within-flower interference, geitonogamy necessarily involves the same processes as outcrossing: pollinator attraction, reward provisioning, and pollen removal.
Because pollen discounting diminishes outcross siring success, interference avoidance may be an important evolutionary force in floral biology.
Large inflorescences attract more pollinators, potentially enhancing reproductive success by increasing pollen import and export.
[78] 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).
[88] Given the tendency of many insect pollinators to forage upwards through inflorescences,[89] protandry may enhance pollen export by reducing between-flower interference.
These effects of protandry on between-flower interference may decouple the benefits of large inflorescences from the consequences of geitonogamy and pollen discounting.
[citation needed] It has been demonstrated experimentally that dichogamy both reduced rates of self-fertilization and enhanced outcross siring success through reductions in geitonogamy and pollen discounting, respectively.
[92] In the moth pollinated orchid, Satyrium longicauda, protandry tends to promote male mating success.
