ADH
The average rate of amino acid replacement (× 10 −^10 /site per year) in this protein for
comparisons between species of the Drosophila genus is 32 (Figure 1.2), about the same as
between genera (33, Figure 1.2 and Table 1.2). However, the rate is 41.9 when
Drosophila species from different subgenera are compared (Table 1.2, row 3), and
decreases to 22.2 and 15.9, when the species compared are from different groups of the
same subgenus, or from the same group, respectively (see rows 2 and 1, Table 1.2). The
rate increases to 260.8× 10 −^10 /site per year, when drosophilid species are compared with
the tephritids Ceratitis or Bactrocera. Thus, the rate for comparisons between different
families is ≈16 times greater than the average rate within Drosophila groups (15.9× 10 −^10 /site
per year). Tephritids and drosophilids diverge about 11 times faster for ADH than for
DDC (23.3× 10 −^10 /site per year), and eight and five times faster than for XDH (32.8× 10
− (^10) /site per year), and SOD (49.4× 10 − (^10) / site per year), respectively. For comparisons
between Drosophila species at various taxonomic levels, the rates of divergence are fairly
similar for these four genes. Besides the extensive differentiation of ADH between
drosophilids and tephritids, ADH-based trees cluster the tephritids closer to the more
distantly related Calyptrata sarcophagids than to the drosophilids, which are, like the
tephritids, Acalyptrata (Brogna et al. 2001).
Two scenarios, which are not mutually exclusive, could account for the ostensibly fast
rate of ADH divergence between Drosophilidae and Tephritidae; namely an accelerated
rate of evolution in the last common ancestor to the Drosophilidae and/or paralogy
resulting from a duplication prior to the divergence of Drosophilidae and Tephritidae.
Evidence for the first scenario comes from the observation that ADH evolves about four
times faster in tephritids (53.1× 10 −^10 /site per year between Ceratitis and Bactrocera,
assuming that the genera diverged at 35 Ma, Beverly and Wilson 1984) than in the saltans
and willistoni groups (12.5× 10 −^10 /site per year), reflecting that the evolutionary rate of
ADH can in effect fluctuate dramatically. In favour of the paralogy scenario is the
observation that Adh has undergone multiple duplication events during evolution (see
Table 1.2 Normalized rates of evolution of ADH, AMD, DDC, GPDH, SOD, and XDH for
increasingly remote lineages of Diptera
The species compared are listed in Figure 1.1. The plus/minus values for myr are crude estimates
of error. Rate values are expressed in units of 10−^10 per site per year. The rates are estimated using
the a values obtained from the dipteran dataset given in Table 1.1.
14 FRANCISCO RODRÍGUEZ-TRELLES ET AL.