The Va/Ba balanced lethal strain: thirty years of research in Drosophila subobscura (1977–2007)

Thirty years ago Diether Sperlich, Tübingen and other researchers published the description of the Va/Ba balanced lethal strain [Sperlich et al. 1977; for the description of the dominant mutations Va and Ba see the report of the department of biometry of the University College, London, (1946) DIS 20: 82]. According to the ISI Web of Knowledge, this paper has been cited 51 times. The availability of the Va/Ba strain signified that Drosophila subobscura became a member of the selected group of species with balanced lethal stocks, as Drosophila melanogaster, Drosophila pseudoobscura, Drosophila persimilis, Drosophila willistoni or Drosophila prosaltans. This was a landmark in the D. subobscura evolutionary research, because this event opened new possibilities for the genetic studies in this species. Since then, it has been possible to analyse the lethal genes (both in natural or experimental populations) and to obtain homokaryotypic chromosomal lines. Sperlich provided this useful balancer stock to several European laboratories, in which research teams worked with this species, which could be considered the European equivalent of D. pseudoobscura. After the publication of the paper, soon appeared in the literature some studies on chromosomal viability and lethal chromosome frequencies in Palaearctic populations (Loukas et al. 1980; Pfriem and Sperlich 1982; Kohonen-Corish et al. 1985; Mestres et al. 1990; Saura et al. 1990, 1998; Andjelkovic et al. 1998; Zivanovic et al. 2000; Zivanovic and Marinkovic 2003). Other papers included the lethal allelism tests which allowed, for instance, to estimate the parameters that define the genetic structure of populations (Begon et al. 1980; Loukas et al. 1980; Mestres et al. 1990; Mestres and Serra 1991; Zivanovic et al. 2000, 2007). In 1978 D. subobscura was detected for the first time in America, in the Chilean population of Puerto Montt and in 1982 it was also found in North America (Port Townsend, Washington). We have analyzed this colonization by using lethal genes. This allowed us to understand some questions of the colonizing event: the magnitude of the founder event, the strong relationship between both colonizations, to estimate the size of the initial sample of colonizers, and to test some hypotheses on the origin of the colonization (Ayala et al. 1989; Prevosti et al. 1989; Mestres et al. 1990, 1992, 1995, 2005; Mestres and Serra 1995; Solé et al. 2000; Zivanovic and Mestres 2000). In American populations several lethal genes persisted in time because they were associated with heterotic chromosomal inversions (Mestres et al. 2001). As the lethal chromosomal lines from America have been maintained in heterokaryotypic condition using the Va balancer chromosome, it has also been possible to sequenciate them for the Odh (Octanol dehydrogenase) gene. These nucleotide sequences have added more information on the colonizing process (Mestres et al. 2004).

Va/Ba strain allows the researchers to obtain a whole chromosome in homokaryotypic condition. This procedure was a cornerstone in the early studies of nucleotide polymorphism in Drosophila species. Since the nineties, several papers have been published dealing on this topic in D. subobscura (Rozas and Aguadé 1990, 1991a,b, 1993, 1994; Rozas et al. 1995, 1999; Khadem et al. 1998; Navarro-Sabaté et al. 1999; Munté et al. 2005). Furthermore, obtaining isochromosomal lines can be useful for other kinds of research (Pfriem 1983; Santos et al. 1986, 2005; Zapata et al. 1986; García-Agustín et al. 1993; Mestres et al. 1998; Fernández-Iriarte et al. 2003). This strain can also be used for studying some chromosomal inversions lethal in homokaryotypic condition (Orengo and Prevosti 1992; Mestres et al. 1998, 2002).

However, although the Va/Ba balanced lethal strain has been an excellent genetic tool, it has some minor pitfalls. One of its O chromosomes carries the O_st arrangement, whilst its homologue presents the X-ray induced and overlapped VIII + 210 inversions. These artificial inversions cover about two-third of the whole chromosomal length (named segment SII). The other fragment carries the naturally occurring overlapped inversions 3 + 4 (segment SI). Thus, if a wild chromosome carries the O₃₊₄ arrangement without any overlapping inversions, crossing-over between the wild type and the Va balancer chromosomes may occur. We found that the frequency of recombination in this segment was 8.033 ± 0.010% (Mestres et al. 1990). For this reason, the study of lethal genes in natural populations is not easy, because depending on which inversions the wild chromosome carries, it is possible to analyze the whole O chromosome (segments SI + SII) or only a fraction of it (segment SII). To overcome this difficulty, Loukas et al. (1980) developed a mathematical method to transform all analysed chromosomes in SII ‘equivalents’. This procedure is also useful to test whether the lethal genes distribute at random along the O chromosome (Mestres and Serra 1999). Another shortcoming is the poor penetrance of the Ba mutation. It has been shown that there are modifiers that affect this mutant, and even some of them have been located (Alvarez et al. 1981, 1990; Alvarez and Zapata 1996). Fortunately, the Va chromosome carries two more genetic markers (ch, cherry and cu, curled) both recessive. Using a combination of crosses with the homokaryotypic chromosomal strain chcu and the Va/Ba it is possible to avoid confusing Ba individuals with wild-type (Mestres et al. 1990). Moreover, this pattern of crosses can be used to investigate simultaneously the chromosomal polymorphism and the lethal genes of natural populations.