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Three decades ago, Todd (1) postulated that wholesale fission of all “mediocentric” (metacentric, submetacentric, and subtelocentric) chromosomes in a complement plays a major role in chromosome evolution. According to Todd, karyotypic fission produces, as a consequence of a single mutational “event,” dramatic differences in the diploid numbers of closely related species (2, 3). A diversity of karyotypes is generated through the random assortment of parent and fissioned homologous chromosomes. Immediate descendants of the fissioned parent would exhibit identical fundamental numbers of functional chromosomal arms but (potentially) very different diploid numbers. Todd saw karyotypic fission events, followed by the accumulation of pericentric inversions, as the driver for explosive speciation in adaptive radiations. He used the label “Karyotypic Fission Theory” to call attention to his implicit rejection of Darwinian gradualism in chromosomal evolution. Whole karyotypic fissioning can (at least theoretically) generate drastically different karyotypes in far fewer steps than are required according to competing explanations of chromosomal evolution, whether based on reciprocal or nonreciprocal chromosomal fission or fusion. Shortly after Todd's article was published, it was dismissed as preposterous by one of the leading theorists of chromosomal evolution, M. J. D. White (4). If chromosomal fissioning occurs only under unusual circumstances, how could an entire karyotype be expected to fission? A serious problem was the lack of a plausible cellular/molecular mechanism. Indeed, even single chromosomal fissioning appeared difficult to explain (refs. 5 and 6; see ref. 7 for review). Some researchers saw fissioning as requiring a ready supply of extra centromeres, perhaps in the form of vestigial chromosomes (Fig. 1a). But the existence of spare vestigial chromosomes had never been demonstrated. Todd (1) proposed another mechanism—centromeric mis-division and subsequent repair (Fig. 1b). White (4) doubted “whether simple breakage through the centromere of a metacentric can produce two …

Kinetochore reproduction theory may explain rapid chromosome evolution