Drosophila

Russo, Claudia A. M., Mello, Beatriz, Frazão, Annelise & Voloch, Carolina M., 2013, Phylogenetic analysis and a time tree for a large drosophilid data set (Diptera: Drosophilidae), Zoological Journal of the Linnean Society 169 (4), pp. 765-775 : 770-771

MAIN DROSOPHILA CLUSTER

The main Drosophila cluster splits into clades A and B ( Fig. 1 ). Clade A includes three lineages. The first is from the Far East ( Japan and Korea), and includes several Hirtodrosophila lineages and a monophyletic Dichaetophora . The second is the Hawaiian drosophilid clade (HD), which includes the Drosophila annulipes Duda, 1924 (SEA) plus Drosophila maculinotata Okada, 1956 ( Japan) cluster. The African Drosophila adamsi Wheeler, 1959 (unclassified) is weakly linked to the remaining members of the second lineage. Finally, the third lineage within clade A is the newly diagnosed subgenus Siphlodora (sensu Yassin, 2013) .

This association between the HD clade and a Far East Asian cluster ( D. annulipes and D. maculinotata ) is particularly interesting. Drosophila maculinotata is currently not assigned to any species group ( Bächli, 2013), but Okada’s original description placed it in the funebris group (i.e. within our subgenus Drosophila clade; Okada, 1956). Later, Okada argued that D. maculinotata actually belongs to the virilis – repleta radiation (i.e. our Siphlodora subgenus clade; Okada, 1988): this hypothesis is more concordant with our findings (see also Katoh et al., 2007). The other member of the cluster that is sister to the HD clade is D. annulipes , which is currently assigned to the immigrans group within subgenus Drosophila . Nevertheless, morphological and karyotypic data indicate that D. annulipes differs from other immigrans species ( Wakahama et al., 1983; Zhang & Toda, 1992). Thus, despite the geographical support for this association, further morphological analyses are necessary.

The monophyly of the HD clade is far from novel (see O’Grady & DeSalle, 2008; O’Grady et al., 2011). Nevertheless, the inclusion of the type species of Scaptomyza , Scaptomyza graminum (Fallén, 1823) , confirms the taxonomic status of this genus (see also Yassin, 2013). The HD clade includes a monophyletic Scaptomyza , which clusters with the Hawaiian Idiomyia . The monophyly of Scaptomyza is strongly supported; among lower ranks, however, only subgenus Bunostoma receives strong support.

The largest drosophilids in the world are grouped in the Hawaiian Idiomyia , which was originally described by Grimaldi (for support, see Yassin 2013). In Idiomyia , the antopocerus and modified-tarsus species groups are joined to form the AMC clade (see O’Grady & DeSalle, 2008), but the support for this grouping is relatively weak. Also, most of the Hawaiian picture-winged species (the adiastola , grimshawi , and planitibia groups) form a cluster, except for Idiomyia gymnobasis (Hardy & Kaneshiro, 1971) ( grimshawi species group), which is sister to all other Hawaiian Idiomyia in our tree. This result is surprising because the Hawaiian picture-winged clade appears to be highly stable (see O’Grady & DeSalle, 2008, and references therein, but also see Van der Linde et al., 2010). Our data set, however, includes only the Ddc sequence for this species; thus, it may not be directly comparable with many other picture-winged species. In a BLAST search, the I. gymnobasis Ddc sequence most closely matches other sequences from the grimshawi species group, as expected. Therefore, the anomalous placement of I. gymnobasis appears to be an artefact related to the missing data in our matrix or to the ML algorithm.

Morphological data support the inclusion of the unclassified Drosophila fluvialis Toda & Peng, 1989 in the Siphlodora clade (see Grimaldi, 1990). Although the species of the robusta group are united in our topology, the monophyletic quadrisetata group is clustered within the robusta group with strong support. Similar results have been reported in a molecular phylogeny using mitochondrial genes ( Wang et al., 2006). Altogether, these results strongly suggest that the quadrisetata and robusta groups form a single clade.

Based on morphological, ecological, and geographical data, Throckmorton (1975) suggested a primary radiation in Eurasia, with secondary radiations in the New World for what he called the virilis – repleta radiation (i.e. our subgenus Siphlodora ). Most of the species in this group breed in rotting plant matter ( Markow & O’Grady, 2006). In our tree, species groups within the subgenus Siphlodora are assembled according to larger geographical areas, following Throckmorton’s proposal. One cluster contains monophyletic Old World species groups ( angor , melanica , and the quadrisetata plus robusta clade), whereas the other cluster includes species groups that mostly inhabit the New World ( annulimana , bromelia, canalinea , mesophragmatica , nannoptera , and repleta ).

Clade B includes the remaining Hirtodrosophila lineages, Mycodrosophila , and the subgenus Drosophila . The Mycodrosophila cluster does not include the type species of the genus, M. poecilogastra , suggesting the need for a taxonomic reassessment of its generic status. In contrast, Throckmorton (1975) grouped Hirtodrosophila and Mycodrosophila with the Hawaiian drosophilids (see also Grimaldi, 1990). Our tree suggests that certain Hirtodrosophila lineages are closer to the Hawaiian drosophilids and subgenus Siphlodora , whereas others are connected to Mycodrosophila and subgenus Drosophila . Unfortunately, our data set did not include the type species of Hirtodrosophila , Hirtodrosophila latifrontata (Frota-Pessoa, 1945) . Therefore, additional data are needed to verify the status of this genus.

The strong bootstrap support for the position of the cosmopolitan type species of Drosophila , D. funebris , is an important finding. The type species of Drosophila belongs to the small funebris group, so that an uncertain phylogenetic placement of this group would preclude a formal review of the genus ( O’Grady & Markow, 2009). Throckmorton considered this species group to be basal within the Drosophila radiation, but our results provide further statistical support for the funebris plus pinicola clade (see also van der Linde et al., 2010; Yassin, 2013). Our results indicate a consistent position for the funebris group, which is closely linked to Drosophila pinicola Sturtevant, 1942 within a North American clade of subgenus Drosophila . Recently, the pinicola group has been assigned to subgenus Siphlodora ( Yassin, 2013) . According to our results, however, the pinicola group clusters with the type species of (sub)genus Drosophila with strong support (92 BP). The critical position of the type species, D. funebris , is now more firmly established, and a phylogenetic taxonomy for most drosophilids is taking shape ( Fig. 1 ; Yassin, 2013).

Figures

Figure 1. Timescale for drosophilids based on a maximum-likelihood (ML) analysis using a concatenated alignment (9917 bp) of six protein-coding nuclear genes. Several monophyletic branches have been collapsed, indicating that all taxa within that taxonomic rank form a cluster. Support values above branches are bootstrap proportions performed on the ML tree; values less than 50 are not shown.