Chromosome studies on two endemic pill-millipedes of the genus Arthrosphaera (Diplopoda: Sphaerotheriida) from the Western Ghats of India

摘要

The diploid chromosome number of pill-millipedes (Class, Diplopoda) varies between 16 (Glomeridae) and 30 (Arthrosphaeridae) with XY pattern of sex determination. The meiotic division stages and karyotype of chromosomes derived from the spermatogonial metaphase of two pill-millipedes Arthrosphaera craspedota and A. lutescens of the Western Ghats of India revealed the diploid chromosome number 30 and 26, respectively. The meiotic chromosome behavior was almost similar in these species. The zygotene in A. craspedota consists of bouquet formation, the diplotene bivalents exhibited characteristic fuzziness and the sex bivalents were conspicuous with terminal chiasma. Deeply stained heterochromatin regions in chromosomes and conspicuous sex bivalents were seen in A. lutescens. Autosomes as well as sex chromosomes were acrocentric with relative length range was 20.6–170.1 in A. craspedota, while 27.6–145.2 in A. lutescens. It is believed that the acrocentric nature of chromosomes in Arthrosphaera was the result of Robertsonian changes, pericentric inversions and polyploidy. Recent studies revealed that the Malagasy-Indian genus Arthrosphaera evolved between 160 and 80 mya (breakup of Madagascar and India). Cytological investigations on endemic Arthrosphaera (A. brandtii) and its sister taxon Sphaeromimus distributed in the Madagascar may provide more insight on the chromosomal evolutionary pattern in pill-millipedes.