Abstract
The family Sciuridae are nearly cosmopolitan in distribution except for Australia and the polar regions. At present squirrels are classified into about 250 species, and more than 40 per cent of all squirrel species are found in Asia. Based on the morphological and ecological characteristics, they are classified into three types:tree squirrels, flying squirrels, and ground-dwelling squirrels. All the three types are distributed in Asia and their taxonomy and phylogeny remain poorly understood. A study of karyotypic evolution of Asian squirrels is of particular interest. However, only small numbers of Asian squirrel species(15% of total Asian species)are karyotypically studied. Furthermore, studies with chromosomal banding techniques appear to have only been performed on the limited species. Although karyotypic data for Asian squirrels are still somewhat limited, the purpose of this paper is to summarize the present status of chromosome studies in Asian squirrels for the better understanding of evolutionary processes in karyotypes. In summary, (1)the diploid number of Asian flying squirrels and tree squirrels was 38-54 with FN = 68-72, and their karyotypes consisted of mostly biarmed chromosomes. This suggests that the karyotype evolution of Asian squirrels could be caused by more complicated chromosome rearrangements including inversion and translocation rather than centric fusion and centric fission. (2)Comparative banded karyotypes exhibited that the homology of banding patterns have been conserved between genera of tree squirrels more than between genera of flying squirrels. (3)Conspicuous secondary constriction(SC) or satellite with long stalk was found exclusively in flying squirrels and their nucleolus organizer regions(NORs) were clearly observed on SC and satellite stalk. Tree squirrels had no chromosome with SC. However, in karyotype of Callosciurus, the satellite with long stalk which corresponded to NORs was recognized, suggesting that Callosciurus is most closely related to flying squirrels of Asian tree squirrels described here.
