Asexual reproduction

Lore & Background

Asexual reproduction encompasses a variety of mechanisms. Prokaryotes reproduce through binary fission, where the parent divides into two genetically identical daughter cells. Eukaryotes may use mitosis for similar fission, and multiple fission occurs in many protists, such as sporozoans and algae, where the nucleus divides several times before cytoplasm separation. Budding, seen in baker's yeast and hydra, produces a smaller daughter cell that matures and detaches. Internal budding occurs in parasites like Toxoplasma gondii. Vegetative propagation in plants forms new individuals without seeds or spores, as in kalanchoe plantlets or strawberry stolons. Spore formation in many multicellular organisms, including fungi and algae, can involve mitosis (mitospores) or meiosis (as in plants). Fragmentation, where a fragment of the parent grows into a new individual, occurs in planarians, annelid worms, sea stars, and lichens.

Reader's Guide

Asexual reproduction is significant because it allows rapid population increase without the need for a mate, enabling organisms to colonize environments quickly and maintain successful genetic combinations. It is the dominant reproductive mode for prokaryotes and many unicellular eukaryotes. In multicellular organisms, it provides an alternative when sexual reproduction is limited, as seen in facultative parthenogenesis in zebra sharks and Komodo dragons. However, asexual reproduction reduces genetic diversity, which can be a disadvantage in changing environments. The article notes that while all prokaryotes reproduce without gametes, lateral gene transfer mechanisms like conjugation provide genetic recombination akin to sexual reproduction. In plants, spore formation via meiosis is part of a sexual life cycle, but is sometimes considered a form of asexual reproduction (agamogenesis) because it does not involve gamete fusion. The legacy of asexual reproduction includes its role in agriculture (vegetative propagation), understanding of clonal populations, and insights into evolutionary trade-offs between genetic uniformity and diversity.

Did You Know?

• Some aphid species use heterogony, alternating between asexual and sexual reproduction at regular intervals.
• In apicomplexans, multiple fission (schizogony) can result in merozoites, sporozoites, or microgametes.
• Most lichens reproduce through fragmentation to ensure new individuals contain both fungal and photosynthetic symbionts.

Fissiparity Across Starfish Genera

Fissiparity in starfish is a remarkably restricted phenomenon, confined to just a handful of genera within the family Asteriidae—specifically Coscinasterias, Stephanasterias, and Sclerasterias—with a separate, independent evolutionary origin in the family Asterinidae. The mechanism involves the central disc splitting into two portions, each of which then regenerates its missing structures. In Coscinasterias tenuispina, fission typically occurs outside the winter months, which are reserved for sexual reproduction, and the presence of multiple madreporites on the undivided individual appears to be a prerequisite for the division to proceed. New arms emerge in groups of four, accompanied by the appearance of additional madreporites. In Sclerasterias, the capacity to divide is restricted to very young individuals, who display a temporary six-armed symmetry before transitioning to the standard five-armed form and permanently losing the ability to fission. In Brazil, only male Coscinasterias individuals have been documented, and fission occurs year-round, peaking in winter.

The Comet and the History of Autotomy

The idea that a severed starfish arm could develop into an entirely new organism has a long scientific history. Six years later, Ernst Haeckel documented that Ophidiaster species shed arms and subsequently developed new discs, arms, madreporites, and mouths on the severed surfaces. In Hawaii, Linckia multifora and Linckia guildingi regularly shed one or more arms that move independently despite being disconnected from both the nervous system and the water vascular system. The detachment is not instantaneous; a small crack first appears on the lower surface of the arm, spreads laterally and upward toward the dorsal side, and then tube feet on both the arm and the body pull the two parts apart over roughly an hour.

Regeneration Mechanics and Timeframes

The regenerative capacity of starfish fragments is both impressive and precisely timed. After an arm is severed, the damaged tissue requires approximately ten days to heal, after which the survival chances of the fragment improve significantly. A replacement arm then grows over the course of several months. In laboratory settings, fragments exceeding one centimeter in length—whether arm tips or central sections bearing wounds at both ends—proved capable of full regeneration, with a new disc and one-centimeter arms taking roughly ten months to form. The first visible sign in the regeneration cycle is the formation of a crescent-shaped ridge at the site of damage. In fission, each offspring must inherit at least one madreporite from the parent, and new arms typically appear in groups of four alongside additional madreporites. In Sclerasterias juveniles, active fission correlates with six arms and four madreporites, but at an unexplained developmental stage, the animal transitions to five arms and a single madreporite, permanently losing the capacity to divide.

Population Dynamics and Ecological Triggers

Field observations reveal fascinating ecological patterns tied to asexual reproduction in starfish. A dense population of Stephanasterias albula at North Lubec, Maine, consisted entirely of small individuals with arm lengths under eighteen millimeters, yet no juveniles were present—suggesting no recent larval recruitment and implying the species may be obligately fissiparous. Fission in this population occurred only during spring and summer, at a rate of once per year or once every two years per individual. In contrast, Brazilian Coscinasterias populations fission throughout the year with a winter peak, and stress factors such as particularly low tides that expose starfish to air appear to trigger the division. Nepanthia belcheri presents a unique reproductive case: as a hermaphrodite, individuals possessing mature female gonads become masculinized after fission, developing male-type gonads. In Hawaii, of fifty Linckia multifora specimens brought to a laboratory, eighteen shed one or more arms within twenty-four hours, and newly severed arms faced high mortality from bacterial infection before their wounds sealed.