Reproductive relationships and degree of synapsis in the polytene chromosomes of the Drosophila buzzatii species cluster

L P B Machado,L Madi-Ravazzi,W J Tadei

doi:10.1590/s1519-69842006000200010

Abstract

The process of speciation occurs through the evolution of any of several forms of reproductive isolation between taxa, including inviability of hybrids. In this work, strains derived from allopatric populations of Drosophila buzzatii cluster species were experimentally crossed in order to evaluate their reproductive and cytogenetic relationships, and to contribute toward understanding the reproductive isolation in this group of sibling species. Although intrastrain crosses were highly fertile, we consider it relevant to discuss the differences in intra- and interspecific fertility and fecundity here. Among 30 interspecific crosses, about 63% were partially or completely sterile. Fifty three percent of interspecific F1 crosses (female and male F1 crossed) were also partially or completely sterile, in contrast to only one out of 24 intraspecific F1 crosses that was partially sterile. An analysis of hybrid polytene chromosomes revealed complete synapsis, except in the microchromosomes (VI) and in the proximal region of the X chromosome. The intraspecific divergence observed in this study and the variable degree of chromosome pairing shown here reveal part of the complexity of the speciation process pertinent to Drosophila buzzatii cluster, which is consistent with different traits studied in this cluster.

Highlights

The study of reproductive isolation patterns regarding species divergence is critical for understanding the speciation process
Most of the intrastrain crosses were completely fertile, except the D72 (Drosophila seriema) and B20 (D. koepferae) strains, which yielded no progeny in the 3rd oviposition period nor in one of the four crosses, respectively (Fig. 2 and Table 2)
It should be noted that experimental conditions in the laboratory differ considerably from those occurring in nature; they may not even occur in nature at all

Summary

Introduction

The study of reproductive isolation patterns regarding species divergence is critical for understanding the speciation process. Such comparative analyses have been conducted in several animal groups (see Lijtmaer et al, 2003 for an example on birds group). Several studies have demonstrated that interspecific hybrids of Drosophila showed incomplete chromosomal synapsis in comparison with chromosomal synapsis inside the species (Cordeiro, 1968; Evgen’ev, 1971; Bicudo, 1979; Madi-Ravazzi & Bicudo, 1992, Madi-Ravazzi et al, 1997, for examples). MadiRavazzi & Bicudo (1992) and Madi-Ravazzi et al (1997) demonstrated a parallel between hybrid asynapsis and postzygotic reproductive isolation in species of the Drosophila repleta group. The divergence in reproductive traits among incipient species has been well documented in this group (Markow, 1981, 1991; Bizzo, 1983; Madi-Ravazzi & Bicudo, 1992; Marin et al, 1993; Madi-Ravazzi et al, 1997; Machado et al, 2002; Markow et al, 2002; Pitnick et al, 2003)

Methods

Results

Conclusion