Evolutionary Perspectives on Germline-Restricted Chromosomes in Flies (Diptera)

In some eukaryotes, germline soma differentiation involves elimination of parts of the genome from somatic cells. The portions of the genome restricted to the germline often contain genes that play a role in development and function of the germline. Lineages with germline-restricted DNA are taxonomically diverse, and the size of the germline-restricted genome varies substantially. Unfortunately, few of these lineages have been studied in detail. As a result, we understand little about the general evolutionary forces that drive the origin and maintenance of germline-restricted DNA. One of the taxonomic groups where germline-restricted DNA has been poorly studied are the flies (Diptera). In three Dipteran families, Chironomidae, Cecidomyiidae, and Sciaridae, entire chromosomes are eliminated from somatic cells early in embryonic development. Germline-restricted chromosomes are thought to have evolved independently in the Dipteran families and their size, number, and transmission patterns vary between families. Although there is a wealth of cytological studies on these chromosomes in flies, almost no genomic studies have been undertaken. As a result, very little is known about how and why they evolved and what genes they encode. This review summarizes the literature on germline-restricted chromosomes in Diptera, discusses hypotheses for their origin and function, and compares germline-restricted DNA in Diptera to other eukaryotes. Finally, we discuss why Dipteran lineages represent a promising system for the study of germline-restricted chromosomes and propose future avenues of research on this topic.