A sex-ratio Meiotic Drive System in Drosophila simulans. I: An Autosomal Suppressor
Figure 2
The Molecular Structure and Evolution of Nmy
(A) An incomplete survey of Nmy/CG14370 alleles found in species of the D. melanogaster subgroup. CG14370 is a single-exon gene in most species, but various inserts are found in D. simulans and D. mauritiana that represent alleles of Nmy. Arrow: transcription orientation; rectangle: transcripts; filled: coding sequence; empty: untranslated region. The status of translation in D. simulans and D. mauritiana is unclear.
(B) Comparison of the three Nmy inserts found in D. mauritiana and D. simulans. For each allele, solid lines indicate the genomic sequence with various sequence components marked with symbols (e.g., an empty triangle for a 93-bp element). For Nmy[sim2041] and nmy[sim1427]. Partial transcripts of various lengths are shown as rectangles (filled: coding sequence as in CG14370 but with earlier termination; dotted: alternative transcripts). Black arrow indicates transcription orientation. Two likely alternatively spliced introns (I and II) were identified. Possible spurious reverse-transcription products were also indicated (curved dotted line, see Text S3 for more details).
(C) Origin of Nmy inserts by retrotransposition. The cDNA of Dox, Nmy[sim2041], and Nmy[mau2791] are compared and the paralogous regions are highlighted (parallelogram; its twisted form for reverse orientation). The positions of three introns of 57, 63, and 63 bp (vertical broken bar with intron size) and an alternative intron of 91 bp (horizontal green bar) from Dox are shown. Nucleotides at the 5′ ends of the two Nmy alleles and the cDNA of Dox are shown; also shown are the 3′ end of Nmy[sim2041]. Eleven nucleotides, TTGTTTAATTT, near the 3′ end of the Dox transcription were duplicated during the retrotransposition event. The insertion site within CG14370 for the retrotranspostion is also shown. The dinucleotide, TA, before the insertion target (^) was also duplicated, but the nearby tetranucleotides TTGT (underlined) might not be related to the other TTGT (framed). The 3′ end sequences from both Nmy alleles fail to match to the extant cDNA of Dox, but the sequence does match to the genomic regions 114 and 225 bp upstream of Dox, respectively (dotted lines with the number of 114 or 225). The left-hand inverted repeat IR' was most likely generated as a secondary duplication after the retrotransposition event.
(D) Both Nmy[sim2041] and Nmy[mau2791] evolved from a common retrotransposed sequence of Dox. Upper: Homologous sequences of 1,467 bp among Dox, Nmy[sim2041], and Nmy[mau2791] were used to construct a star phylogeny with the number of single nucleotide substitutions as well as the three insertions/deletions of 1–3 nucleotides (D3, D3, and D1) mapped on the branches. Lower: Homologous sequences of 345 bp among the two inverted repeats of Nmy[sim2041], Nmy[mau2791], and Dox were used to construct another star phylogeny. The four inverted repeat sequences are essentially identical except for a 6-bp deletion (D6) in IR' of Nmy[sim2041], and two nucleotide substitutions in IR'' of Nmy[mau2791].