The chromosome cycle of a primitive cecidomyiid--Mycophila speyeri.

Abstract

1. The chromosome cycle of the primitive cecidomyiid Mycophila speyeri has three features also found in more specialized cecidomyiids: a large number of chromosomes in the germ-line, chromosome elimination from future somatic cells, and chromosome elimination in spermatogenesis. However, sexual oogenesis lacks the unusual features found in other cecidomyiids, and instead there is normal chromosome pairing and segregation. The studies on male and female meiosis suggest that sexual progeny begin development with fewer chromosomes than their parents possessed, and therefore a compensatory increase in chromosome number probably occurs during early cleavage in sexual progeny. 2. Two cytologically distinguishable patterns of chromosome elimination in cleavage are described, one of which has not been reported previously. 3. The evolutionary origin of the cecidomyiid chromosome cycle is discussed in the light of these results. It is concluded that the essential innovation in the ancestral group was an increase in chromosome number, possibly by polyploidy, and that chromosome elimination in cleavage and spermatogenesis made possible the perpetuation of this elevated chromosome number. Modifications of oogenesis are viewed as secondary adaptions not present in the early cecidomyiid stock.