Evolution of the Mojavensis Cluster of Cactophilic Drosophila with Descriptions of Two New Species

Abstract

The mojavensis cluster of the repleta species group of Drosophila (Drosophilidae: Diptera) consists of three species. One is newly described as D. navojoa. A second species, described here as D. arizonae, replaces D. arizonensis, which has become a junior subjective synonym for D. mojavensis, the third species in the cluster. A phylogeny of the three species is presented, based on chromosomal inversions, morphology, and the ability to produce hybrids. Breakage points are assigned for all inversions, and male genitalia are figured; 186 crosses were made from 225 possible combinations among 15 geographic strains from the southwestern United States, Mexico, and Guatemala. It is confirmed that D. mojavensis and D. arizonae are very closely related and shown that D. navojoa is more distantly related in regard to all criteria. This relationship is supported by the geographical positions of the ancestral gene sequences in each species, which show a sequential northwest movement (D. navojoa----D. arizonae----D. mojavensis) from southern Mexico to southern California and northern Arizona. The relationship is also supported by the fact that D. navojoa breeds in Opuntia cactus, an ancestral behavior, whereas the other two species breed chiefly in Stenocereus cacti, a derived behavior. The possible role of this host plant shift in speciation is discussed.

The Molecular Basis of Host Adaptation in Cactophilic Drosophila: Molecular Evolution of a GlutathioneS-Transferase Gene (GstD1) inDrosophila mojavensis

“…The mitochondrial pattern contradicts the previous proposed model of the speciation of D. mojavensis and D. arizonae. Ruiz et al (1990) proposed that D. mojavensis diverged from D. arizonae in the Baja Synonymous sites include changes within both the coding and the noncoding regions. Syn., synonymous; nonsyn., nonsynonymous.…”

“…), and barrel (Ferocactus cylindraceus), respectively (Fellows and Heed 1972;Ruiz and Heed 1988). D. mojavensis has been proposed (Ruiz et al 1990) to have originated in Baja California, utilizing a Stenocereus cactus (possibly agria), and then migrated up the peninsula and colonized Catalina Island and the Mojave Desert, shifting cactus hosts in the process. A subsequent colonization (and host shift) from Baja to Sonora established the present-day mainland Sonora Desert population.…”

Evolutionary cytogenetics of the Drosophila buzzatii species complex

“…2 Ruiz recently was able to show that 3a was in fact present in D. arizonae (Ruiz et a!., 1990). This encouraged the re-examination of chromosome 3 in the martensis cluster to determine whether the inversion there was indeed 3a.…”

Early events in speciation: Polymorphism for hybrid male sterility in Drosophila

“…In sons of the reciprocal cross, no male sterility originally was observed (16)(17)(18)(19). Subsequently, however, a strain of D. mojavensis was discovered on Santa Catalina Island, California, and when females of this population were crossed to D. arizonae males they were reported to produce sterile sons exclusively (20). Because it appears that hybrid male sterility depends on the source of the maternal population, the responsible factors clearly are not yet fixed in D. mojavensis, providing an unusual opportunity to examine the genetic basis for a postzygotic incompatibility arising early in speciation.…”