Sexual Selection: A Very Short Introduction by Marlene Zuk & Leigh W. Simmons

Author:Marlene Zuk & Leigh W. Simmons
Language: eng
Format: epub
Tags: Biology, Evolution, Genetics & Genomics, Life Sciences, Science
ISBN: 9780191084416
Publisher: Oxford University Press
Published: 2018-06-28T03:00:00+00:00

The genetic basis for this unusually complex set of male morphs was published in 2016, when scientists used sophisticated genomic techniques to discover that a single ‘supergene’ composed of 125 contiguous genes determines whether a male is a satellite or a faeder. The supergene appears to have arisen via an inversion, in which a stretch of DNA is flipped around, that occurred about 4 million years ago, a long time for such variation to have persisted. Two copies of the supergene are lethal, but the composition of the genes in individuals that have a single copy determines their morph. That first inversion seems to have differentiated between the territorial and other males. Then, a portion of the supergene seems to have inverted again some 500,000 years ago, yielding another variant. Males that have only the ancestral version of the stretch of DNA become the territorial morph, while males that carry one copy of the original inverted supergene become faeders, and those with the newest version are satellites. That a single stretch of DNA could influence male appearance, behaviour, and fertility is extraordinary, and appears to be due to the kinds of genes in the supergene, some of which affect levels of hormones such as testosterone.

A surprisingly similar example of genetically predetermined male strategies can be seen in an animal that is quite different from the ruffs, a small marine isopod, Paracerceis sculpta, that lives within cavities of sponges in the intertidal zones along the Gulf of California in the Pacific Ocean (Figure 15). Like the ruffs, male P. sculpta come in three forms. Large alpha males are territorial. They occupy a sponge and allow females to enter, but attempt to exclude other males. Medium-sized males, the beta form, are female mimics, like the faeders. The beta males enter the sponges unchallenged by the alphas, and mate with the females that are already inside. Finally, the tiny gamma males slip undetected into the sponges, avoiding the other types of males that are present. All of these morphological and behavioural differences are controlled by a single gene with multiple alleles. When only one female is present inside a sponge, the alpha male sires nearly all of the young. But when two or three females are in a sponge, the other two forms can obtain some of the matings. Unlike elephant seals, who make the best of a bad job when they are young, the alternative morphs of these marine isopods are maintained in the population because, on average, they have equal reproductive success.

15. Male isopods come in three varieties, all pursuing different reproductive strategies.

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