Abstract
SUMMARYThe karyotype of Z. nasicum is described. It consists of 2n=20 ♂ (XY)-20 ♀ (XX), with a pair of large metacentric autosomes, a metacentric X and an acrocentric Y, the remaining autosomes being acrocentric. Both the metacentric autosomal pair and the XY♂- XX ♀ sex mechanism are present in all the species cytologically examined of Aleuas and Zygoclistron except an undescribed species of Aleuas where the metacentric autosome dissociated into two acrocentric chromosomes. So, an ancient and common origin for both the fusions involved is suggested.The coiling mechanism in the XY pair during the first meiotic division is of an unusual type in Z. nasicum.
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