The genome size, basic chromosome number, and ploidy of genus Pennisetum varies among species. We isolated one new repetitive sequence each from P. orientale (PoSat1) and P. villosum (PvSat1) and two new repetitive sequences from P. setaceum (PsSat1 and PsSat2). The unit length of these repetitive sequences was 152–192 bp and no homology was seen between them. These sequences were located in the subtelomeric regions in a limited number of species. PoSat1 was highly repeated in P. orientale and P. flaccidum, and moderately repeated in P. villosum and P. setaceum, while other species had a lower number of this repeat. PvSat1, PsSat1, and PsSat2 were species-specific. These subtelomeric repeat sequences were mostly amplified in species with small chromosomes. We also examined the distribution of the repetitive centromeric sequence isolated from P. glaucum. This sequence tended to be repeated in species with large chromosomes. Here, we discuss the evolution of these repetitive sequences in Pennisetum.

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The mammalian X and Y chromosomes were originally a homologous chromosome pair. The accumulation of deleterious mutations and subsequent inactivation and loss of Y-linked genes led to the genetic degeneration of the Y chromosome over evolutionary time, approximately 300 million years (Myr). In modern humans, only around 45 Y-linked genes remain. Most of the remaining Y-linked genes have acquired functions essential for males, e.g., sex determination or spermatogenesis. Because of the active role the Y chromosome plays in male function, it has been proposed that the loss of human Y-linked genes is inexorable and may lead to the extinction of the entire species. A contrary opinion suggests that the remaining Y-linked genes are strictly conserved through purifying selection. Recent comparative genomic analyses of the Y chromosome sequences of three primates (human, chimpanzee, and rhesus macaque) revealed high conservation of human Y-linked genes, at least over the past 25 Myr. In rodents, however, there are species in which Y-loss has actually occurred. Two species of Ryukyu spiny rats and one species of mole vole have XO/XO sex chromosome constitutions because they lack Y chromosomes altogether. Furthermore, the mammalian sex-determining gene, SRY, was also lost upon disappearance of the Y chromosome in these species. These XO species reveal that Y-loss in humans would not necessarily lead to the extinction of men, and that there are several possible trajectories for the future evolution of the Y.

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