The continuity of chromosomes between one division of the nucleus and the next was a major topic of discussion in the early days of the science of cytogenetics. Wilson (1907 Wilson ( , 1909) ) was studying this question in the insect Metapodius terminalis, the leaf-footed plant bug, when he made a remarkable discovery: he found B chromosomes, which he called supernumeraries. In Metapodius the standard chromosome complement consists of 2n = 20 autosomes, plus a pair of sex chromosomes, and in

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... ition to that, certain individuals also carried from 0 to 6 of what we now call supernumerary B chromosomes (Bs). The properties described by Wilson for these supernumeraries were as follows: the number varies between individuals, with some having none; there is no pairing between these supernumeraries and the standard A chromosome set; no obvious phenotypic effects; no