Asexual reproduction

Although asexual reproduction is very common in organisms such as bacteria, protists, fungi, and plants, it is rarer among animals, especially among the more complex animals. In simple animals, such as sponges (phylum Porifera), the polyps of hydra, jellyfish, corals, and sea anemones (phylum Cnidaria), and many flatworms (phylum Platyhelminthes), asexual reproduction is common.

Sponges can reproduce asexually when fragments break off and become established as new individuals or when mature sponges produce gemmules, overwintering buds that are produced and released by many freshwater and a few marine sponges. Cnidarian polyps frequently reproduce by budding. The buds start as small regions of less-differentiated tissue that differentiate into a new polyp. These new polyps can separate from the original to form new individuals or can remain attached and form colonies. The largest colonies to be produced asexually by budding are those produced by the reef-building corals. In some Cnidaria, the polyps release free-floating forms of the organism called medusae (jellyfish). The medusa form of the life cycle, which was formed asexually, reproduces sexually when it matures. Many species of flatworms reproduce asexually by fragmentation. When these worms are cut into fragments, most fragments can regenerate their missing parts and, thus, form several organisms from the original worm. Others, like many trematode flukes, asexually reproduce by polyembryony. In this mode of reproduction, larval flukes form many juveniles of the next larval stage internally from the mature larva’s own cells. The immature larvae that are produced, which are all genetic clones of the original larva, will be released to continue the life cycle.

Parthenogenesis

In higher organisms, asexual reproduction is much less frequent and far more complex. Rotifers (phylum Rotifera) are small aquatic organisms with rows of cilia around their mouths that seem to rotate as they beat. Rotifer populations are usually either mostly or entirely female. This happens because rotifers usually reproduce asexually by parthenogenesis. In this type of reproduction, the females produce diploid eggs instead of the haploid eggs that are needed for sexual reproduction. The diploid eggs have the same genes as the mother and are, thus, clones and mature into adults identical to their mother. In some rotifers, asexual reproduction is the only form of reproduction, and thus, all of the organisms in these species are female. In other species, males are only produced during times of environmental stress, and sexual reproduction only occurs then.

Like rotifers, many populations of aphids (phylum Arthropoda), a common plant pest, are entirely or mostly female during parts of the breeding season. In spring and early summer, aphids reproduce parthenogenetically, with females producing diploid eggs that develop into adult female aphids. These eggs and the adults formed from them are clones of the original aphid. In late summer and early fall, the aphids reproduce sexually, producing haploid eggs that can be fertilized by haploid sperm from males.

Even among the very complex vertebrates, a few organisms reproduce asexually. Among the best studied are the whiptail lizard (genus Aspidoscelis), native to the deserts of the American Southwest and the northwestern part of Mexico, and the gecko, found on some tropical islands of the Pacific. These lizards exist in both sexually reproducing and parthenogenetic forms. Genetic study of their chromosomes has shown that the parthenogenetic species were first formed as diploid or sometimes triploid hybrids of two sexually reproducing forms. The hybrids could not undergo normal meiosis because different chromosomes inherited from each parent could not align properly. Thus, these lizards cannot reproduce sexually. They do, however, produce diploid or triploid eggs that can be triggered to start reproduction. The progeny that are produced are exact genetic duplicates of their mothers—clones. Komodo dragons, the largest vertebrates known to regularly reproduce using asexual means, can also reproduce by sexual means depending on their environment. Other animals that can reproduce asexually include zebra sharks, aphids, wasps, ants, starfish, marbled crayfish, blacktop sharks, and Burmese pythons.

Research continued into asexual reproduction in the twenty-first century, and further discoveries were made. Zoologists at the San Diego Zoo Wildlife Alliance discovered in 2021 that two captive California condors, a critically endangered species, were born from unfertilized eggs in captivity. Later, a crocodile was documented asexually reproducing, laying fourteen eggs. Scientists suggest this similarity supports the theory that birds and crocodiles have a common evolutionary origin. This discovery may also indicate that some animals have found a new method of species survival, which may have applications for species preservation.

In other studies, scientists successfully modified the genetics of fruit flies and mice, allowing them to reproduce asexually. However, the limitations of genetic diversity that result from asexual reproduction can negatively impact the ability of some species to adapt to environmental changes. Additionally, harmful gene mutations occurring in multiple generations can harm the species' longevity. However, research into the asexual reproduction of clonal raider ants offers insight into preserving the diversity of genes.

With asexual reproduction, organisms do not have to waste energy in sexual activity. However, the energy savings is not without a price. With the exception of the rotifers, no populations of asexually reproducing organisms have very long histories, evolutionarily speaking. Unlike most sexually reproducing populations with varying degrees of diversity, all members of an asexually reproducing population are identical, except for mutations that arose after the population’s inception. This lowered diversity makes the population much less likely to be able to adapt to change. More or less water, higher or lower temperatures, introduction of parasites, or disease could more easily wipe out the entire population.

Principal Terms

Clone: an organism that is genetically identical to the original organism from which it was derived

Diploid: having two of each chromosome; a normal state for most animals

Haploid: having one of each chromosome; a normal state for animal gametes

Parthenogenesis: a form of asexual reproduction where the young are derived from diploid or triploid eggs produced by the mother without any genetic input from a male

Triploid: having three of each chromosome; an abnormal state that is unable to produce normal haploid gametes

Bibliography

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