Myoblast fusion in Drosophila

Abstract

The body wall musculature of a Drosophila larva is composed of an intricate pattern of 30 segmentally repeated muscle fibers in each abdominal hemisegment. Each muscle fiber has unique spatial and behavioral characteristics that include its location, orientation, epidermal attachment, size and pattern of innervation. Many, if not all, of these properties are dictated by founder cells, which determine the muscle pattern and seed the fusion process. Myofibers are then derived from fusion between a specific founder cell and several fusion competent myoblasts (FCMs) fusing with as few as 3-5 FCMs in the small muscles on the most ventral side of the embryo and as many as 30 FCMs in the larger muscles on the dorsal side of the embryo. The focus of the present review is the formation of the larval muscles in the developing embryo, summarizing the major issues and players in this process. We have attempted to emphasize experimentally-validated details of the mechanism of myoblast fusion and distinguish these from the theoretically possible details that have not yet been confirmed experimentally. We also direct the interested reader to other recent reviews that discuss myoblast fusion in Drosophila, each with their own perspective on the process [1-4]. With apologies, we use gene nomenclature as specified by Flybase (http://flybase.org) but provide Table 1 with alternative names and references.

Fig.1. A model representing proteins required for Drosophila myoblast fusion

Colored boxes represent gene products in which the loss-of-function phenotype is associated with myoblast fusion. White boxes and dashed arrows represent protein interactions that have been observed in S2 cells. Transparent boxes denote proteins in which overexpression of a dominant negative transgene impacts myoblast fusion, but null alleles have no apparent defects. Proteins without boxes have the potential for a role in myoblast fusion, based on the interactions or phenotypes of their vertebrate counterparts. Solid arrows represent interactions observed in the embryonic musculature. In all cases, see text for details.