Comparison of the sexual behaviour gene fruitless between D. melanogaster and two sympatric Hawaiian species, D. heteroneura and D. silvestris.
Davis, Terence. Department of Pathology, University of Wales College of Medicine, Heath Park, Cardiff CF14 4XN, Wales. e.mail firstname.lastname@example.org
The Hawaiian Drosophila complex probably consists of more than 1,000 species (Kaneshiro, 2000), including some of the most morphologically diverse species in the genus, yet current evidence suggests they arose from one or two introductions to the Hawaiian chain roughly 25-40 million years ago (Kambysellis et al., 1995; Carson, 1997). An example of recent speciation is the split between the sympatric species D. heteroneura and D. silvestris. Both these species are in the planitibia species group, are found only on the Island of Hawaii, and are believed to have diverged from a common ancestor within the last 400,000 years, the estimated age of the island of Hawaii (Carson, 1970). This would entail a single founder event from an older island. An alternative theory is that these species diverged from different parental species of the planitibia group before migrating to Hawaii, a situation requiring two migration events. In either case the common ancestor(s) presumably migrated from one of the neighbouring Islands to the northwest. The species with the most recent common ancestor(s) are most probably D. planitibia from Maui or D. differens from Molokai. For more information on the possible origins of D. heteroneura and D. silvestris see Carson (1970, 1997), Ahearn et al. (1974), and Kaneshiro (2000). Kaneshiro (1976) has proposed a role for sexual behaviour and sexual selection in the evolution of the Hawaiian Drosophila, particularly the role of female mate choice (Ohta, 1978; Kaneshiro, 1980). Many of the Hawaiian species have evolved elaborate sexually dimorphic characteristics, e.g., the broadened head observed in D. heteroneura: females of this species are believed to prefer males with wide heads (Boake et al., 1997).
Table 1: Comparison of intron-exon sizes (bp) between the three species.
aType: c is an exon common to most or
all transcipts, A, B, C, D, E are 3’ terminal exons
A total of 21,524bp of the fru locus from D. heteroneura have been sequenced in three pieces. The largest contig is 17,532bp in length (AF051662) and includes exons III to IX as enumerated by Davis and Ito (2001). Exon X is included in 3,431bp of adjacent genomic sequence (AF051664) and exon XI is a cDNA sequence of 561bp (AF051669). The proteins encoded by these sequences begin at the BTB domain and include the type A, B, and C protein types (Davis et al., 2000a). In addition protein types D and E have been deduced by comparison with the D. melanogaster sequence (Table 1); however, these have not been found as cDNAs. For the definition of the various protein types see Usui-Aoki et al., (2000) and Davis and Ito (2001). In D. silvestris 8,030bp of this locus have been sequenced in three pieces (Davis et al., 2000b). The first piece of 2,404bp includes exons III to V (AF051665), the second piece of 3,039bp includes exons VI to VIII (AF051666) and the third piece of 2,587bp includes exon IX (AF051667). The proteins encoded begin at the BTB domain and include the type A, C and D protein types (Table 1), although only type A has so far been found as a cDNA (Davis et al., 2000b).
male specific peptide encoded by exons I and II in D. melanogaster has not yet been found in the Hawaiian Drosophila, and none of the putative promoter sequences are known.
The majority of the sequence for each species is intronic. The intron and
exon sizes and the transcripts for the Hawaiian species and D. melanogaster
are summarised in Table 1.
The male specific peptide encoded by exons I and II in D. melanogaster has not yet been found in the Hawaiian Drosophila, and none of the putative promoter sequences are known. The majority of the sequence for each species is intronic. The intron and exon sizes and the transcripts for the Hawaiian species and D. melanogaster are summarised in Table 1.
a gross level the fru genes in the
three species are precisely conserved in that there are the same number of
exons in the same order (Table 1), and the intron-exon boundaries are the
same (not including exons I and II). The known and deduced transcripts are
also well conserved. The actual exon sizes have some small differences: when
compared to D. heteroneura exon
VII is longer in D. silvestris,
and all exons except III and IV are slightly different lengths in D.
melanogaster (Table 1). Exon VII encodes the type C terminal exon.
The other full length terminal exon known for the Hawaiian species is exon
IX (type A) and this is longer than that in D. melanogaster.
On a gross level the fru genes in the three species are precisely conserved in that there are the same number of exons in the same order (Table 1), and the intron-exon boundaries are the same (not including exons I and II). The known and deduced transcripts are also well conserved. The actual exon sizes have some small differences: when compared to D. heteroneura exon VII is longer in D. silvestris, and all exons except III and IV are slightly different lengths in D. melanogaster (Table 1). Exon VII encodes the type C terminal exon. The other full length terminal exon known for the Hawaiian species is exon IX (type A) and this is longer than that in D. melanogaster.
|Table 2. Nucleotide changes in the fru gene between D. heteroneura and D. silvestris
anucleotide position refers to the position in the D. heteroneura sequence Accession number AF051662. Amino acids are indicated in brackets for coding sequence changes (when only a single amino acid is given the nucleotide change is a synonymous one). The gaps in the numeration indicate the three different D. silvestris sequences.
The exons in D. melanogaster vary considerably in the level of amino acid conservation with the Hawaiian species. The conservation varies from 100% for exons III and IV that encode the BTB domain, to 46% for exon IX. The combined conservation for the exons common to the various transcripts (exons III, IV, V, VI and VIII) is 78%. The 3’ terminal exons show amino acid conservation of 72%, 63%, and 86% (for types B, C and E respectively). The 3’ end of the type D transcript (exon VIIIa) has only two amino acids for each species (Gly and Glu).
A striking observation is that there are only four coding sequence changes (one amino acid change) for exon IX (975bp coding) between the Hawaiian species. This is one fifteenth of the amino acid changes in one third of the protein. Exon IX encodes the type A Zinc finger sequence. The first third of this exon is partially conserved in D. melanogaster and the Zn finger region is highly conserved (Davis et al., 2000a,b). The rest of the exon (approximately half the length), however, is completely unconserved. The overall conservation is 46%. I have looked at this exon in a related Hawaiian species, D. mimica (AF051673). The 1011bp of coding sequence for this species has 110 nucleotide differences (11%) compared to the D. heteroneura se-quence (not shown), and the protein homol-ogy is high with only 36 amino acid changes. The majority of these changes are in the region of high variability seen between D. melanogaster and D. heteroneura. Thus the majority of this exon appears to be a rapidly diverging sequence and would appear to be a useful region for future phylogenetic studies in the Hawaiian Drosophila. A phylogenetic representation of the exon IX sequences using ClustalW is given in Figure 1. Interestingly, the two different alleles from D. mimica are less well conserved than the D. heteroneura and D. silvestris sequences.
Unfortunately the close similarity of the fru sequences between D. heteroneura and D. silvestris may not shed light on the two alternate theories of the origins of these species. Although at first glance the data suggest that these species are the result of a very recent speciation event (presumably after the migration to Hawaii of the parental species), the sequences may have converged due to sequence introgression through the natural hybridisation known in these species (Carson et al., 1989; Kaneshiro, 2000). However, the data do indicate the close relationship between the two species.
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Figure 1. ClustalW analysis of the exon IX coding sequences. D. mimica1 and D. mimica2 are different alleles for this species.