Academic journal article Genetics

Population Genetics of the Developmental Gene Optomotor-Blind (Omb) in Drosophila Polymorpha: Evidence for a Role in Abdominal Pigmentation Variation

Academic journal article Genetics

Population Genetics of the Developmental Gene Optomotor-Blind (Omb) in Drosophila Polymorpha: Evidence for a Role in Abdominal Pigmentation Variation

Article excerpt

ABSTRACT

The developmental gene optomotor-blind (omb) encodes a T-box-containing transcription factor that has multiple roles in Drosophila development. Previous genetic analyses established that omb plays a key role in establishing the abdominal pigmentation pattern of Drosophila melanogaster. In this report we examine patterns of omb nucleotide variation in D. polymorpha, a species that is highly polymorphic for the phenotype of abdominal pigmentation. Haplotypes at this locus fall into two classes that are separated by six mutational steps; five of these mutational events result in amino acid changes. Two lines of evidence are consistent with a role for omb in the abdominal pigmentation polymorphism of D. polymorpha. First, we find that haplotype classes of omb are correlated with abdominal pigmentation phenotypes, as are microsatellite repeat numbers in the region. Second, tests of selection reveal that the two haplotype classes have been maintained by balancing selection. Within each class there is a significantly low amount of diversity, indicative of previous selective sweeps. An analysis including D. polymorpha's closest relatives (members of the cardini group) provides evidence for directional selection across species. Selection at this locus is expected if omb contributes to variation in abdominal pigmentation, since this trait is likely of ecological importance.

THE gene optomotor-blind (omb) is one of the essential "toolkit" genes used to coordinate development in diverse animal taxa (CARROLL et al. 2001). The omb locus encodes a T-domain transcription factor (PFLUGFELDER et al. 1992a,b) that plays key patterning roles in the central nervous system (BRUNNER et al. 1992; PFLUGFELDER et al. 1992b; POECK et al. 1993) and leg and wing imaginal discs (GRIMM and PFLUGFELDER 1996; LECUIT et al. 1996; NELLEN et al. 1996) of Drosophila melanogaster. The T-domain is a highly conserved region that functions in DNA binding (PFLUGFELDER et al. 1992a; KISPERT and HERRMANN 1993; KISPERT et al. 1995). The T-box family is named after its founding member, the mouse Brachyury (T) gene (BOLLAG et al. 1994).

In addition to its early developmental roles, omb is necessary for patterning the pigment band in each adult abdominal segment: in the developing pupa, omb is expressed at the posterior edge of each abdominal segment in a stripe that prefigures the pigment band of the adult (Kopp and Duncan 2002). omb loss-of-function clones lose pigmentation and overexpression of omb leads to expansion of the pigment band (KOPP and DUNCAN 1997). The pattern and extent of the pigment pigband is a character that is highly variable in the genus (reviewed in True 2003; Wittkopp et al. 2003a), differing among species groups (GARCIA-BELLIDO 1983; GOMPEL and CARROLL 2003), between closely related species (HOLLOCHER et al. 2000), between sexes within a species (KOPP et al. 2000), and geographically within a species (HEED 1963; LEE 1963; ROBERTSON et al. 1977; OHNISHI 1985). The control of pigmentation in D. melanogaster by omb suggests that omb may also be important in specifying this natural variation within and between species of Drosophila.

To explore omb's role in abdominal pigmentation variation, we examined patterns of nucleotide variation at the omb locus in natural populations of a South American species of fruit fly, D. polymorpha. D. polymorpha is a member of the cardini group, members of which exhibit extensive abdominal pigmentation variation, both within and between species (STALKER 1953; HEED 1963; HEED and RUSSELL 1971; HOLLOCHER et al. 2000). D. polymorpha is one of the most polymorphic members of this group, with individuals that vary from having almost completely pigmented abdomens to having almost completely unpigmented abdomens (DA CUNHA 1949; MARTINEZ and CORDEIRO 1970). Although abdominal pigmentation is environmentally determined in some species (DAVID et al. 1990; GIBERT et al. 1996), early crossing experiments determined that the abdominal pigmentation polymorphism in D. …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.