Chromosomes and sex determination in 13 araneid spiders of North-Eastern India

The diploid chromosome number of 13 species belonging to the family Araneidae (Araneae) ranges between 16 and 25. The sex-determining mechanism seems to be X₁X₂X₃X₄O in one species; X₁X₂X₃O in three species and X₁X₂O in the remaining 9 species. The X₁X₂X₃X₄O, sex-determining mechanism is reported for the first time in spiders. All the chromosomes are with terminal centromeres. C-band preparations in 5 species suggest procentric localization of constitutive heterochromatin in all the chromosomes, though distally located C-band positive blocks are also visible in a few chromosomes.It is suggested that chromosomal evolution in this family has taken place by gradual reduction in diploid number through tandem/centric fusion followed by pericentric inversion in autosomes and nondisjunction/duplication of one X, with subsequent loss of homology in sex chromosomes.