Abstract
Although crocodilians have attracted enormous attention in other research fields, from the cytogenetic point of view, this group remains understudied. Here, we analyzed the karyotypes of eight species formally described from the Alligatoridae family using differential staining, fluorescence in situ hybridization with rDNA and repetitive motifs as a probe, whole chromosome painting (WCP), and comparative genome hybridization. All Caimaninae species have a diploid chromosome number (2n) 42 and karyotypes dominated by acrocentric chromosomes, in contrast to both species of Alligatorinae, which have 2n = 32 and karyotypes that are predominantly metacentric, suggesting fusion/fission rearrangements. Our WCP results supported this scenario by revealing the homeology of the largest metacentric pair present in both Alligator spp. with two smaller pairs of acrocentrics in Caimaninae species. The clusters of 18S rDNA were found on one chromosome pair in all species, except for Paleosuchus spp., which possessed three chromosome pairs bearing these sites. Similarly, comparative genomic hybridization demonstrated an advanced stage of sequence divergence among the caiman genomes, with Paleosuchus standing out as the most divergent. Thus, although Alligatoridae exhibited rather low species diversity and some level of karyotype stasis, their genomic content indicates that they are not as conserved as previously thought. These new data deepen the discussion of cytotaxonomy in this family.
Highlights
Unlike other vertebrates that underwent substantial diversification, extant crocodilian species have maintained morphological and ecological similarities for almost 100 million years (Myr) [1,2,3,4]
The collection sites, number, and sex of individuals are summarized in Figure 1 and Table 1
The 2n for all Caimaninae species equaled 42 for both sexes. Their karyotypes were composed of 24a + 18m/sm chromosomes in C. crocodilus and C. latirostris; 28a + 14m/sm chromosomes in C. yacare, P. palpebrosus, and P. trigonatus, and 32a + 10m/sm chromosomes in M. niger
Summary
Unlike other vertebrates that underwent substantial diversification, extant crocodilian species have maintained morphological and ecological similarities for almost 100 million years (Myr) [1,2,3,4]. The order Crocodylia is a useful model for biogeographic studies, as its species show a circumtropical distribution, with at least one extant representative in each continent, except for Europe and Antarctica [9,10] Such circum-oceanic distribution, combined with the ancient age and distinct phylogenetic position, make crocodilians an attractive model to understand evolutionary and biogeographic characteristics of ancient vertebrates, including the fact that they demonstrate the past dispersal events of many vertebrate lineages. Their biogeography is still a mystery, some recent studies proposed that a relatively recent trans-Atlantic crossing, from Africa to the New World and from
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