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The Y chromosomal fertility genes of Drosophila melanogaster have been studied for almost 80 years (1) but the secrets of their function have remained elusive. Even their number and their location on the Y chromosome were recently revised (2, 3), as the Y chromosome poses serious difficulties for genetic mapping due to the absence of marker genes. It is now accepted that two fertility genes (ks-i and ks-2) reside in the short arm and four (kl-l, -2, -3, and -5) [contrary to the originally supposed five (4)] reside in the long arm of the Y chromosome of D. melanogaster. A new area of research on the fertility genes started when Meyer et al. (5) discovered that the activity of these genes is accompanied by the formation of large lampbrush loops in primary spermatocyte nuclei. Spermatocytes are the only cell type in which the Y chromosomal fertility genes are active. Soon it became clear that Y chromosomal lampbrush loops are a universal feature of spermatocytes in the genus Drosophila (6). In D. melanogaster, these lampbrush loops are relatively small and difficult to distinguish. Therefore, most of the recent studies of the fertility genes have been carried out in Drosophila hydei. While D. hydei has advantages for biochemical work, the genetics of this species is only poorly developed (for a recent review see ref. 7). For D. hydei, it has been concluded that each Y chromosomal lampbrush loop is related to a single complementation group (8). By hybridization experiments it has been shown that the Y chromosome codes for transcripts of repetitive DNA sequences specifically expressed in spermatocytes (9, 10). In both D. melanogaster and D. hydei, however, attempts to identify protein products related to the expression of the fertility genes have not been very successful (11, 12). However, evidence was presented (13) that the D. melanogaster Y chromosome affected a dynein-like protein, although these experiments could not distinguish between the presence of a structural gene or a regulatory effect. Also, extensive studies of the DNA composition of the lampbrush loopforming fertility genes did not provide any evidence for protein coding of these genes. In D. melanogaster, only satellite DNAs have been found as transcribed constituents of the lampbrush loops (14). The molecular structure of some of the lampbrush loops has been investigated in much more detail in D. hydei (for review, see ref. 15). These loops, similar to those of D. melanogaster, are composed of a satellite-like type of DNA present in small tandem repeat clusters, which are interspersed by members of other repetitive DNA sequence families that for a large part, if not entirely, are defective transposable elements (16-21). In these studies, no DNA sequences coding for functional proteins were discovered, although considerable stretches of DNA were sequenced. The report of Gepner and Hays (22) in this issue of the Proceedings that the ATP-binding domain of a presumably functional dynein gene is found associated with the kl-5 fertility gene of D. melanogaster therefore provides solid evidence for the presence of a proteincoding gene not only on the Y chromosome but also related to one of the fertility genes. The assignment to the kl-5 fertility gene is based on the use of a deficiency of the region of the Y chromosome accommodating the kl-5 gene. Some indirect earlier evidence for the presence of potentially protein-coding genes came from my laboratory (21). Leoncini had isolated a series of temperature-sensitive alleles of the fertility genes of D. hydei. However, as conditional tRNA mutants are known, the possibility remained that the conditional mutations may be caused by defects other than in protein structure. We have not been able to identify the target sequences in the DNA responsible for the conditional mutations. The observations of Gepner and Hays imply that the mutants recovered by Leoncini may indeed be mutations in protein-coding genes. The results reported by Gepner and Hays raise the important question of how the Y chromosomal dynein relates to the fertility genes. If it does, what are the consequences in the context of our knowledge of the molecular structure of the transcription units in the lampbrush loops? The data provided by Gepner and Hays imply that a defective dynein gene results in male sterility due to defects in development caused by the absence of dynein. However, they do not give any information concerning a relationship between the transcription of the dynein gene and the transcription units appearing as lampbrush loops. By Miller spreading experiments it was earlier shown that the lampbrush loops formed by the fertility genes of D. hydei are giant transcription units (23, 24). From indirect cytogenetic evidence, similar large sizes have later been postulated for the fertility genes of D. melanogaster. The most direct conclusion would be to assume that such a large transcription unit, if coding for dynein, has a large number of introns interspersed between a series of small exons or has few exons spaced by very large introns. The results communicated by Gepner and Hays give information neither on the genomic length of the Y chromosomal dynein gene nor with respect to its exon-intron structure. It will be of much interest to see further data answering these two questions. But it is not even clear yet whether the dynein gene has any relationship with the lampbrush loop-forming region of the fertility gene kl-5. If it has, it might as well be positioned at the very beginning or at the end of the transcription unit in a structure with few (or no) introns. But is it situated in the loop-forming region at all or is it located outside the lampbrush loop transcription unit? The latter possibility seems incompatible with the finding that only single complementation groups can be assigned to the various fertility genes identified. However, we do have to keep in mind that the absence of markers in the Y chromosomes raises substantial difficulties for analysis of the genetic fine structure of genes on this chromosome and that we may end up with unpredicted discoveries. Some genetic data also seem incompatible with all current models made for the structure and function of the fertility genes. It has repeatedly been claimed that the Y chromosomal genes have very high mutation rates and that this can be explained only by the presence of physically large genes. However, in an exonintron model as discussed before, this

Conventional protein coding genes in the Drosophila Y chromosome: is the puzzle of the fertility gene function solved?