α-methyl-dopa (Amd; E.C.4.1.1.-), (4) glycerol-3-phosphate dehydrogenase (Gpdh; E.C.1.1.1.
8), (5) glucose-6-phosphate dehydrogenase (G6pd; E.C.1.1.1.49), (6) phosphogluconate
dehydrogenase (Pgd; E.C.1.1.1.44), (7) superoxide dismutase (Sod; E.C. 1.15.1.1), (8)
triosephosphate isomerase (tpi; E.C.5.3.1.1), and (9) xanthine dehydrogenase (Xdh; E.C.1.1.1.
204) (Table 1.1 and Figure 1.1). Six genes (1–4, 7, 9) have been analysed in 34 species of
Diptera, comprising representatives of the families Drosophilidae and Tephritidae
(‘Diptera’ dataset in Table 1.1; see also Figure 1.1) and the last six (genes 4–9) in 95
species that include representatives of the three multicellular eukaryote kingdoms, i.e.
animals, plants, and fungi (‘Global’ dataset, Table 1.1; see Figure 1.1).
The enzymes encoded by the nine genes (i.e. ADH, AMD, DDC, GPDH, G6PD, PGD,
SOD, TPI, and XDH) are globular soluble proteins generally composed of two identical
subunits with approximate molecular masses of 28, 54, 54, 40, 60, 53, 15, 25, and 145
KDa, respectively. The encoded enzymes are oxidoreductases, except for DDC and AMD,
which are carboxylases, and TPI, which is an isomerase.
ADH is a member of the insect-type, or ‘short-chain’ dehydrogenase/reductase family
(SDR). In Drosophila melanogaster the Adh coding sequence is interrupted by two introns. The
protein consists of two domains for binding the coenzyme (NAD) and the substrate
(Benyajati et al. 1981). The crystal structure and reaction mechanisms of ADH have been
elucidated in Scaptodrosophila lebanonensis (Benach et al. 1998). The enzyme plays a key role
in adaptation to substrates undergoing alcoholic fermentation. It has been intensively
investigated in Drosophila from a variety of perspectives (Powell 1997).
Table 1.1 Likelihood-ratio test carried out on the amino acid data sequences for various genes
The null hypothesis (H 0 ) is compared with a hypothesis (H 1 ) that removes the assumption indicated
on top. Log-likelihood values are obtained assuming the topology in Figure 1.1 The probability of
obtaining the observed value of the likelihood ratio statistic (−2logΛ) if H 0 were true, with degrees
of freedom (d.f.) indicated, is <10−^6 for all tests. Poisson: Poisson model; JTT-F: Yang et al. (1998)
model; JTT-F+dG assuming discrete gamma distributed rates at sites. The α values are obtained
with the JTT-F+dG model.
8 FRANCISCO RODRÍGUEZ-TRELLES ET AL.