Chromosomes and spermatozoa of the great apes and man
Seuanez, Hector N.
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The chromosome complement of four species phylogenetically related to man, the chimpanzee (Pan troglodytes), the pygmy chimpanzee (Pan paniscus), the gorilla (Gorilla gorilla), and the orangutan (Pongo pygmaeus) have been analysed with chromosome banding techniques and compared to the human chromosome comple¬ ment. This has shown remarkable homologies between species, and presumed mechanisms of chromosome evolution have been proposed. Chromosome heteromorphisms in the great apes have been compared to those found in human populations, and most of them affected the distribution or the amount of constitutive heterochromatin and/or brilliantly fluorescent material, a situation comparable to man where such variations have been established as chromosome polymorphisms. However, a balanced polymorphic structural rearrangement involving large segments of euchromatic material has been found in two populations of orangutan. This rearrangement consisted of two pericentric inversions, one inside the other, comprising an unusual kind of chromosome polymorphism in mammalian populations. Moreover, it showed that pericentric inversions, the most probable chromosome rearrangements in the phylogeny of the chromosomes of man and the great apes, might not necessarily be restricted by infertility barriers, but may spread successfully in the population. The patterns of late replication of the chromosomes of the great apes and man have been compared, using BUdr as a thymidine substitute in the cell cycle. This has shown remarkable similarities in the patterns of late replication between species, and, as in the human chromosomes, most regions of late replication in the chromosomes of the great apes corresponded to areas of positive G-banding. Q-, C- and G-banding as methods of demonstrating chromosome homologies between these species have been analysed in relation to the content of highly repeated satellite DNAs in man and homologous sequences in the great apes. This has shown that the banding patterns are not informative about these sequences, and that they must reflect a degree of chromosome organization due to DNA packaging rather than DNA composition. Finally, the phylogeny of the chromosomes of man and the great apes has been reconstructed in view of the findings presented in this work and of previous data in the literature. In this study, man and gorilla resembled each other more closely than to any of the other species studied, a finding that is contrary to the generally held view that man and the chimpanzee are the two most closely related species. Comparative studies of the spermatozoa of the great apes and man were undertaken and showed that man was not unique in producing pleiomorphic spermatozoa, since this feature was also present in the gorilla. Moreover, the morphology of human and gorilla spermatozoa resembled each other so closely that on morphological grounds it was impossible to distinguish the spermiogram of these two species. Fluorescent ("f") bodies were detected in the spermatozoa of the African apes, although the distribution of such bodies did not resemble that of human spermatozoa, where the Y chromosome is usually visible. An analysis of the haploid DNA content of the great apes and man w®3 undertaken by estimating the total dry mass of the sperm head in these species. Man showed the lowest DNA content, whilst the gorilla showed the highest; this latter latter species also showed a higher variability in the haploid DNA content than all other species, including man. Diploid spermatozoa were also detected in the gorilla, in proportions similar to those found in man. These findings on spermatozoa are indicative of a closer relationship between man and gorilla than between man and the other hominoid apes. Moreover, the heteromorphism of spermatozoa in both human and gorilla semen samples makes it unlikely that clothing induced hyperthermia is the cause of pleiomorphic spermatozoa in man.