Evolution of secondary cell number and position in the Drosophila accessory gland

Abstract

In animals with internal fertilization, males transfer gametes and seminal fluid during copulation, both of which are required for successful reproduction. InDrosophilaand other insects, seminal fluid is produced in the paired accessory gland (AG), the ejaculatory duct, and the ejaculatory bulb. TheD.melanogasterAG has emerged as an important model system for this component of male reproductive biology. Seminal fluid proteins produced in theDrosophilaAG are required for proper storage and use of sperm by the females, and are also critical for establishing and maintaining a suite of short- and long-term postcopulatory female physiological responses that promote reproductive success. TheDrosophilaAG is composed of two main cell types. The majority of AG cells, which are referred to as main cells, are responsible for production of many seminal fluid proteins. A minority of cells, about 4%, are referred to as secondary cells. These cells, which are restricted to the distal tip of theD.melanogasterAG, may play an especially important role in the maintenance of the long-term female post-mating response. Many studies ofDrosophilaAG evolution have suggested that the proteins produced in the gland evolve quickly, as does the transcriptome. Here, we investigate the evolution of secondary cell number and position in the AG in a collection of eight species spanning the entire history of theDrosophilagenus. We document a heretofore underappreciated rapid evolutionary rate for both number and position of these specialized AG cells, raising several questions about the developmental, functional, and evolutionary significance of this variation.