In the Journals - Reproductive isolation in Drosophila hybrids

Of the many questions in evolutionary biology, the genetic basis of reproductive isolation between species and subspecies is a pretty hot topic. Drosophila pseudoobscura is a new world Drosophila species that has been used in evolutionary biology studies for many years.  This paper looks at the genetic basis of the hybrid sterility and segregation distortion seen in crosses between two subspecies, D. pseudoobscura pseudoobscura (referred to as "USA") and D. pseudoobscura bogotana (referred to as "Bogota"). It's a nice illustration of the impact of the 12 Drosophila genome sequences now available - D. pseudoobscura was the second Drosophila species to have its genome sequenced.

There is only partial reproductive isolation between these subspecies - male progeny from Bogota females crossed with USA males are virtually sterile (though when aged, they apparently yield offspring, though with a distorted sex ratio indicative of segregation distortion).  The female siblings are fertile, as are the offspring of a cross performed between UAS females crossed with Bogota males.  The two subspecies therefore obey Haldane's Rule.

This partial reproductive isolation permitted the investigators to localise (one of) the genes responsible.  Previous work had indicated the crucial region in the Bogota strain lay near to the eye colour mutant sepia, which lies on the right arm of the X chromosome.  The Bogota chromosome was introgressed into the USA genome through 28 generations (even with the short life cycle usual in Drosophila, this is quite a lengthy experiment).  Bogota females were crossed to USA males and female progeny recovered - these are hererozygous for the se marked USA X chromosome and the se⁺ Bogota X chromosome.  Recombinants bearing se were identified by crossing these females to Bogota males. With every set of two generations, the region surrounding se that was reduced by random recombination in the females (this is indicated by the white section flanking se in the diagram).

175 introgression lines were tested by crossing to USA males - all progeny were fertile (and with no indication of distorted segregation), indicating that the loci for these effects were tightly linked to se, and to each other (and that both reproductive isolation and segregation distortion might result from the same locus). The region was molecularly defined using microsatellites and SNPs to approximately 20kb and five genes.  One of these genes, GA19777, turned out to have eight non-synomymous changes whenthe sequences from Bogota and USA were compared.

GA19777 is a bit of a cumbersome name - later in the paper the authors name it Overdrive (Ovd), so I'll refer to at as this from here.  Ovd encodes a small DNA binding protein, but its biological function remains unclear.  A complex set of transgenic experiments in which Ovd transgenes were swapped between subspecies and the effects on hybrid sterility and segregation distortion revealed that

• Ovd  effects on sterility and SD result from the encoded protein, not some cis effect - the transgene functions irrespective of the chromosome that carries it.
• The SD effect of Ovd^BOG seems to result from single nucleotide changes that have occurred since the two subspecies split.
• SD can be suppressed by recessive autosomal suppressors - indicating Ovd is part of a complex set of genetic interactions.

For me, the paper has two interesting features - one is the technical aspects of how the genome sequence of D. pseudoobscura has facilitated the detailed characterisation of a single locus which has profound effects, not on viability or morphology but on reproductive isolation between evolutionarily new subspecies.  The other is that we have the opportunity to get to grips with the biology of at least one example of reproductive isolation between taxa.  One presumes that there may be different molecular mechanisms involved (the other example in Drosophila would be the reproductive isolation between the two sibling species Drosophila melanogaster and Drosophila simulans).

 
N. Phadnis, H. A. Orr (2008). A Single Gene Causes Both Male Sterility and Segregation Distortion in Drosophila Hybrids Science DOI: 10.1126/science.1163934