Size classes of the head and flagellum of Drosophila spermatozoa

Abstract

Among the different species and spermatozoan types of the genus Drosophila there is immense variation in the whole length of the spermatozoon (ca. 55 to 15,000 µ) and in the length of its head (ca. 7.5 to 60µ). The width of the head and tail is of the order of 0.5 µ. There are either one, two, or three principal size classes of spermatozoa in every male, the number being species-specific. More than one size class in individual animals will be referred to as polymegaly. Both the head and flagellum exhibit a closely related polymegaly. The correlation between head and flagellum length within a size class is zero. According to the species, Drosophila spermatozoa in the testis are known to occur in bundles of, for example, 128 spermatozoa, each bundle derived from a single spermatogonium. Polymegaly is found to exist between but not within bundles. Polymegaly cannot, therefore, be due to meiotic segregation of factors affecting spermatozoan size. An hypothesis that polymegaly arises by omission of one or more of the successive waves of mitotic division during formation of the bundle is apparently untenable. In species with polymegaly, spermatozoa of a length intermediate between those of the size classes typical of the species are virtually never released from the testis. All size classes of spermatozoa are motile and reach the female genital tract but seem not to be randomly disposed between the female storage organs. Much of the variation between size classes both within and between species of the genus seems to reflect an harmonious underlying pattern of a restricted number of size modules that follow a geometric progression. The modular size or sizes would be determined by the genome of the species, perhaps also by the sex-chromosome complement in particular and by the rearing temperature.