Evolutionary history of the genus Sus inferred from cytochrome b sequences

The systematic status of species belonging to the genus Sus has been a matter of debate for decades (Groves, 1981). According to a recent review of its taxonomy, there are a total of 7 living species (Sus scrofa, Sus salvanius, Sus barbatus, Sus verrucosus, Sus celebensis, Sus philippensis, Sus cebifrons) and approximately 22 subspecies (Groves and Grubb, 1993), to which one recently recognized species, Sus bucculentus (Groves et al., 1997) and many new subspecies (Groves, 1997) should be added. South-East Asia (SEA) may be considered the homeland of the genus, as 6 out of the 8 species are endemic to the area, the only exception being the pygmy hog (S. salvanius), nowadays present only in the north of India, and S. scrofa, which is distributed worldwide (and not limited to SEA). In particular, the relationship between SEA species was investigated, but the evidence presented until now is still not conclusive. Morphological analyses point toward the identification of two main lineages, one leading to scrofa/celebensis/philippensis and the other to barbatus/verrucosus (Groves, 1997), but molecular evidence does not support the same grouping, but merges scrofa, barbatus and verrucosus (Lucchini et al., 2005). Recent findings have further complicated this issue rather than solving it, highlighting a substantial level of polyphyly among some of these species (see Larson et al., 2005). There is also great uncertainty about the temporal scale of Sus evolution, mainly because of a general lack of representative fossil records. For example, the origin of the genus probably dates back to near the Miocene/Pliocene boundary, around 5 Mya1 (Randi et al., 1996), but its divergence from its closest genus, Phacochoerus, based only on molecular data, ranges from 5 to 15 Mya (Randi et al., 1996). This great uncertainty also characterizes estimates about the separation between European and Asian lineages of S. scrofa, spanning from 56,000 (Kim et al., 2002) to 500,000 years (Giuffra et al., 2000), depending on the marker and mutation rate used. The major aim of the present study was to provide a more comprehensive description of the timing of evolution of the genus Sus and of the phylogenetic relationships among its various species. We thus sequenced mitochondrial cytochrome b from some specimens belonging to S. scrofa, S. celebensis and S. barbatus. This gene was selected as it was the most represented marker available for Sus in the GenBank. We further tested the performance of the cytochrome b as a molecular barcode, in order to check its ability to recognize potential hybrid populations such as that inhabiting New Guinea, proposed to be a cross between S. celebensis and S. Scrofa (Groves, 1981; but see Larson et al., 2005, for a different opinion).