On the nature of eclectic species—a tiered approach to genetic compatibility in the desmidMicrasterias thomasiana

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Abstract
The applicability of the concept of the biological species to the Desmidiaceae is investigated by interbreeding trials between sympatric and allopatric populations of the ubiquitous morphospecies Micrasterias thomasiana. A four-tiered system of testing is adopted, in which the degree of genetic relationship between strains is progressively adduced by the relative success in each of the series of tests, culminating in F1 interfertility and gene flow in backcrossing of F1 offspring to parental clones. The results indicate that sexual compatibility is more complex than simple heterothallism, and involves relative sexuality and probably a system akin to a gene complex with polygenic modifiers. The probable existence of syngens, or biological species, within the morphospecies is acknowledged, one of which has a considerable measure of intercontinental genetic compatibility within morphological boundaries consistent with the nomenclatural species but distinct from closely similar taxa. For an operational taxonomy of desmids the credo of the biological species is declined in favour of an holistic, multidisciplinary enquiry and an eclectic species.