Sequential hermaphroditism
Sequential hermaphroditism is one of the two types of hermaphroditism, the other type being simultaneous hermaphroditism. It occurs when the organism's sex changes at some point in its life. A sequential hermaphrodite produces eggs and sperm at different stages in life. Sequential hermaphroditism occurs in many fish, gastropods, and plants. 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.
In animals, the different types of change are male to female, female to male, and bidirectional. Both protogynous and protandrous hermaphroditism allow the organism to switch between functional male and functional female. Bidirectional hermaphrodites have the capacity for sex change in either direction between male and female or female and male, potentially repeatedly during their lifetime. These various types of sequential hermaphroditism may indicate that there is no advantage based on the original sex of an individual organism. 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 first 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.
Zoology
Hermaphroditic fishes are almost exclusively sequential—simultaneous hermaphroditism is only known to occur in a few fishes, such as the Rivulid killifish Kryptolebias marmoratus and hamlets. Teleost fishes are the only vertebrate lineage where sequential hermaphroditism occurs.Protandry
In general, protandrous hermaphrodites are animals that develop as males, but can later reproduce as females. 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:- Protandrous sequential hermaphroditism: Early reproduction as a pure male and later reproduction as a pure female.
- Protandrous hermaphroditism with overlap: Early reproduction as a pure male and later reproduction as a pure female with an intervening overlap between both male and female reproduction.
- Protandrous simultaneous hermaphroditism: Early pure male reproduction and later reproduction in both sexes.
Protandrous examples
Protandry occurs in a widespread range of animal phyla. In fact, protandrous hermaphroditism occurs in many fish, mollusks, and crustaceans, but is completely absent in terrestrial vertebrates.Protandrous fishes include teleost species in the families Pomacentridae, Sparidae, and Gobiidae. A common example of a protandrous species are clownfish, which have a very structured society. In the species Amphiprion percula, 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 reproductive male being the second largest. The rest of the group is made up of progressively smaller males that do not breed and have no functioning gonads. If the female dies, in many cases, the reproductive male gains weight and becomes the female for that group. The largest non-breeding male then sexually matures and becomes the reproductive male for the group.
Other protandrous fishes can be found in the orders Clupeiformes, Siluriformes, and Stomiiformes. Since these groups are distantly related and have many intermediate relatives that are not protandrous, it strongly suggests that protandry evolved multiple times.
Phylogenies support this assumption because ancestral states differ for each family. For example, the ancestral state of the family Pomacentridae was gonochoristic, indicating that protandry evolved within the family. Therefore, because other families also contain protandrous species, protandry likely has evolved multiple times.
Other examples of protandrous animals include:
- The Platyctenida order of comb jellies. Unlike most ctenophores, which are simultaneous hermaphrodites, Platyctenida are primarily protandrous, but asexual reproduction has also been observed in some species.
- The flatworms Hymanella retenuova.
- 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, 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.
- Members of the shrimp genus Lysmata perform protandric simultaneous hermaphroditism where they become true hermaphrodites instead of females. During the "female phase", they have both male and female tissues in their gonads and produce both gametes.
Protogyny
Protogynous examples
Protogyny is the most common form of hermaphroditism in fish in nature. About 75% of the 500 known sequentially hermaphroditic fish species are protogynous and often have polygynous mating systems. In these systems, large males use aggressive territorial defense to dominate female mating. This causes small males to have a severe reproductive disadvantage, which promotes strong selection of size-based protogyny. Therefore, if an individual is small, it is more reproductively advantageous to be female because they will still be able to reproduce, unlike small males.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 a mating pair is the male, while the smaller is the female. In most cases, females and immature males 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, 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 to six years before changing sex since all California sheephead are born female.
Bluehead wrasses begin life as males or females, but females can change sex and function as males. Young females and males start with a dull initial-phase coloration before progressing into a brilliant terminal-phase coloration, which has a change in intensity of color, stripes, and bars. Terminal-phase coloration occurs when males become large enough to defend territory. 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 allows these males to compete with the larger territorial male.
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. 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, Sparidae, Synbranchidae, Labridae, Scaridae, Pomacanthidae, Gobiidae, Lethrinidae, and possibly others.
- The intertidal isopod Gnorimosphaeroma oregonense.
- Protogyny sometimes occurs in the frog Rana temporaria, where older females will sometimes switch to being males.
Ultimate causes