Taxonomic boundaries, phylogenetic relationships and biogeography of the Drosophila willistoni subgroup (Diptera: Drosophilidae)

Abstract

The Drosophila willistoni subgroup represents a complex with varying taxonomic levels. It encompasses D. willistoni and its five sibling species: D. equinoxialis, D. insularis, D. paulistorum, D. pavlovskiana and D. tropicalis. Of these, D. equinoxialis, D. tropicalis and D. willistoni present differentiation at subspecific level, whereas D. paulistorum represents a superspecies, formed by six semispecies. Despite this taxonomic and evolutionary complexity, many of these semi and subspecific taxa have not yet had their phylogenetic status tested in an explicitly molecular study. Aiming to contribute to the understanding of the evolution of this challenging group, we analyzed nucleotide sequences from two mitochondrial and four nuclear datasets, both individually and simultaneously, through different phylogenetic methods. High levels of incongruence were detected among partitions, especially concerning the mitochondrial sequences. As this incongruence was found to be statistically significant and robust to the use of different models and approaches, and basically restricted to mitochondrial loci, we suggest that it may stem mainly from hybridization-mediated asymmetrical introgression. Despite this, our nuclear data finally led to a phylogenetic hypothesis which further refines several aspects related to the willistoni subgroup phylogeny. In this respect, D. insularis, D. tropicalis, D. willistoni and D. equinoxialis successively branched off from the willistoni subgroup main stem, which recently subdivided to produce D. paulistorum and D. pavlovskiana. As regards the semispecies evolution, we found evidence of a recent diversification, which highly influenced the obtained results due to the associated small levels of genetic differentiation, further worsened by the possibly associated incompletely sorted ancestral polymorphisms and by the possibility of introgression. This study also raises the question of whether these semispecies are monophyletic at all. This reasoning is particularly interesting when one considers that similar levels of reproductive isolation could be attained through infection with different Wolbachia strains.

BuT2 Is a Member of the Third Major Group of hAT Transposons and Is Involved in Horizontal Transfer Events in the Genus Drosophila

“…

F ig . 5.—Scheme of phylogenetic relationships of different Drosophilidae species groups employed in this study based on several works (Lewis et al 2005; Robe et al 2005; Kopp 2006; Robe, Cordeiro, et al 2010; Robe, Loreto, et al 2010; Oliveira et al 2012). Numerous species that do not have BuT2 were omitted and only the number of species tested is shown.

…”

Mar, a MITE family of hAT transposons in Drosophila

“… Evolutionary relationships between the willistoni group species, based on [25] , and the results obtained from the PCR and Dot blot screenings. …”

“…This could be indicative of horizontal transfer between species, a common process in TE evolution [22]. However, the species involved are very closely related, and some levels of incongruence were found between different phylogenies of the willistoni subgroup, which suggests that saturation, introgression and perhaps incompletely sorted ancestral polymorphisms due to rapid radiation may have occurred [25]. Considering that Mar is a multiple copy sequence, the Mar phylogeny supports the view that the origin of this MITE occurred after the separation of the willistoni and bocainensis subgroups, but before the subgroup willistoni speciation that began approximately 5.7 Mya [25].…”

Thecopiaretrotransposon and horizontal transfer inDrosophila willistoni

“…D. willistoni has a large geographical distribution, which extends to areas from Florida to Argentina (Spassky et al, 1971 ;Dobzhansky & Powell, 1975). Strains from any part of this distribution do not show incipient reproductive isolation, except for samples from Lima, Peru, which are considered a subspecies called D. willistoni quechua (Ayala & Tracey, 1973 ;Robe et al, 2010). However, the existence of endemic chromosome inversions suggests some level of population restructuring in this species .…”