Karyotypes of freshwater crocodiles (Crocodylus siamensis), salt water crocodiles (crocodylus-porosus) and the interspecific hybrids by conventional and banding techniques
1999
Wiwat Chawananikun | Duangsamon Suwatthana | Wiraphong Koyakun | Sumittra Watthanodon | Chutharat Sukkhai (Chulalongkorn Univ., Bangkok (Thailand). Faculty of Veterinary Science)
Karyotypes of Siamese freshwater crocodiles (Crocodylus siamensis), salt water crocodiles (Crocodylus porosus) and interspecific hybrids of F1 and F2 were studied. The diploid chromosome number of the freshwater crocodile was 30. The saltwater crocodiles which had 2n=34. The karyotype of F1 interspecific hybrids (2n=32) revealed a combination of chromosome complement from each species. In F2 hybrids, three different karyotypes were found depending on the types of mating. Backcrossing of F1 with the freshwater crocodile produced the hybrids with 2n=31 whereas backcrossing of F1 with the saltwater crocodile produced 2n=33. The interse mating (F1*F1) produced 2n=32 consisting of 5 homologous pairs of large metacentric, 6 acrocentric, 6 submetacentric and 10 small metacentric chromosomes. However, these six karyotypes have the same fundamental number (NF) of 58. Comparative analysis of karyotypes made from both species and their hybrids showed that the fusion of acrocentric chromosomes to produce bi-armed chromosomes plays a major role in the production of different karyotypes. The first and second pairs of acrocentric chromosomes have become the third pair of a large metacentric pair and the third and the fifth have become the first pair of existing submetacentric chromosomes. By using NOR's banding technique, the secondary constriction on the fourth pair in the saltwater crocodiles was marked by a prominent dark band. NOR's bands were found in both male and female crocodiles. In fresh water crocodiles, they were on the only one pair of acrocentric chromosomes whereas in saltwater crocodiles, they were on the fourth pair of acrocentric chromosomes. Results from the NOR's banding technique also supports the hypothesis concerning the fusion of acrocentric chromosomes during the course of evolution.
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