The length of the sperm nucleus in Drosophila obscura group species is depending on the total length of the sperm

Abstract

Summary Males of the Drosophila obscura species group develop sperm of more than one length class. Also the lengths of the nuclei are different. When the total sperm length and the nucleus length were subject to a cluster analysis, it was shown that in D. subobscura and in D. affinis two classes of sperm exist. In D. azteca three classes were found. In the analyzed species the lengths of short sperm are more similar than the lengths of long ones. For example, the short sperm of D. subobscura and D. affinis have the same length while the long sperm of the two species are significantly different. This fact supports the idea that in evolution there is a tendency towards an increase in the length of sperm. But according to our data, in species with sperm heteromorphism preferentially the length of the long class is increased. In the two species D. subobscura and D. affinis, the size of the sperm nuclei corresponded to the total length of sperm. Long sperm had longer nuclei than short sperm. In D. azteca only for the length classes of short and medium sperm the same was found, but in the class of the longest sperm of D. azteca the lengths of nucleus and sperm were not correlated. In this class the total length of sperm varied while the length of the nuclei was rather constant with a mean value of 495 μm and a maximum of 631 μm. We assume therefore that the longest nuclei in this class have reached their upper limit of elongation.