Role of Chromosome Changes in Crocodylus Evolution and Diversity

The karyotypes of most species of crocodilians were studied using conventional and molecular cytogenetics. These provided an important contribution of chromosomal rearrangements for the evolutionary processes of Crocodylia and Sauropsida (birds and reptiles). The karyotypic features of crocodilians contain small diploid chromosome numbers (30~42), with little interspecific variation of the chromosome arm number (fundamental number) among crocodiles (56~60). This suggested that centric fusion and/or fission events occurred in the lineage, leading to crocodilian evolution and diversity. The chromosome numbers of Alligator, Caiman, Melanosuchus, Paleosuchus, Gavialis, Tomistoma, Mecistops, and Osteolaemus were stable within each genus, whereas those of Crocodylus (crocodylians) varied within the taxa. This agreed with molecular phylogeny that suggested a highly recent radiation of Crocodylus species. Karyotype analysis also suggests the direction of molecular phylogenetic placement among Crocodylus species and their migration from the Indo-Pacific to Africa and The New World. Crocodylus species originated from an ancestor in the Indo-Pacific around 9~16 million years ago (MYA) in the mid-Miocene, with a rapid radiation and dispersion into Africa 8~12 MYA. This was followed by a trans-Atlantic dispersion to the New World between 4~8 MYA in the Pliocene. The chromosomes provided a better understanding of crocodilian evolution and diversity, which will be useful for further study of the genome evolution in Crocodylia.

Chromosome polymorphisms in natural populations of the South American grasshopper Sinipta dalmani

Six populations of Sinipta dalmani from the provinces of Buenos Aires and Entre Rios (Argentina) were analyzed. The populations of "El Palmar" National Park (Entre Rios) were polymorphic for pericentric inversions in pairs M4 and M7 and for a centric fusion involving pair M5 and the X chromosome. The M4 inversion remained similar over time and the karyomorphic frequencies did not depart from those expected according to the Hardy-Weinberg equilibrium. The analysis of chiasma frequency and distribution showed clear intra- and interchromosome effects of the different chromosome rearrangements. Both inversions and centric fusions were related with total or partial crossing over restriction in heterozygous condition, leading to a genetic differentiation between rearranged and non-rearranged chromosomes. The chromosome polymorphisms analyzed herein were associated with an increase in the number of terminal chiasmata both in the rearranged chromosomes (heterozygous centric fusion and homozygous M4 inversion) and in the other chromosomes (M4 inversion). Our results showed that the chromosome polymorphisms in S. dalmani may be associated with a significant decrease in genetic recombination, which may explain in part their maintenance in some areas of its geographical distribution.