Molecular Systematics and Cospeciation

Over the last several years the lab has been involved in molecular studies concerning the family Rhinonyssidae.  As in many parasitic organisms the morphological evolution has proceeded by simply reducing their phenotypic features, presumably to be less antagonistic to the host.  But from a taxonomic view this makes the designation of species and higher taxa problematic.  To address this difficulty the lab has developed several techniques to obtain DNA sequences from individual rhinonyssid nasal mites specimens.  Four gene regions, two mitochondrial and two nuclear, have been developed for phylogenetic reconstruction in nasal mites: mitochondrial 16S ribosomal RNA gene (16S rRNA), mitochondrial cytochrome b gene (Cyt b), Internal Transcribed Spacer region of the nuclear rRNA gene complex (ITS), and the nuclear divergent domain III (D3) of the large subunit ribosomal RNA (28S rRNA) gene.  These genes are discussed for their utility in mite studies by Navajas and Fenton (2000).  Additionally, some work has been published on rhinonyssid ITS and 16S rRNA genes by Rojas et al. (2001, 2002) and ITS by Ubeda et al. (2003).

In addition, two gene regions have been developed for closely related species.  The first is the mitochondrial Cytb gene.  So far we only have a 380 bp sequence from specimens of Tinaminyssus melloi and Ptilonyssus sairae, but it appears to be very useful for closely related species, and for species delimitation.  The other gene section that has proven useful for relatively closely related nasal mite taxa is the ITS region of the rRNA gene complex.  The lab has sequence for a region encompassing 473 bp for specimens in the genera Tinaminyssus, Rhinonyssus, and Ptilonyssus.  The emphasis of our work on this gene has been to examine specimens that are considered closely related species or the same species based on morphology.  In particular, the lab has focused on the Ptilonyssus sairae species complex.  This group of nasal mites has long been a contentious group among nasal mites workers, with researchers considering this to be a speciose group of 12 species, or a morphologically variable group of just four species (Pence and Casto, 1976).  Most recent workers (Spicer, 1977a, b, 1978; Wilson, 1980) have agreed with the conclusions of Pence and Casto (1976) that it is a highly variable group with few species, since they showed that the characters used to differentiate species are so variable that they are not of systematic use.  However, our molecular studies have revealed a dramatically different picture for the evolution of this group (see figure below).  Not only does this appear to be a speciose group, but the results suggest that every species of bird host has its own species of Ptilonyssus "sairae" mite.  This conclusion is based on the sequence of multiple P. sairae mites from the same host species, and then comparing them to mites collected from a different host species.  What is revealed is that there is little or no genetic variation for mites found within the same host species, but substantial genetic variation for mites examined from different host species.  For example, the mites from the Sparrow hosts have genetic variation of between 6-8% for pairwise comparison, while mites from the Sparrows to Warbler host comparison have about a 17% difference, and finally the comparison of the mites from the Sparrows and Warbler hosts to the outgroup P. pirangae from the Tanager host give about a 22% uncorrected sequence divergence.  As a point of reference, the published results of Rojas et al. (2002) using the ITS region show levels of variation around 1-8% divergence among morphologically different species in the genus Sternostoma, and about 13-44% divergence among different morphological species in the genus Tinaminyssus.  This is what would be expected if each bird species has its own species of mite, since the ITS gene is part of the nuclear ribosomal RNA complex which undergoes concerted evolution.  Consequently, little or no genetic variation is found within species, but substantial variation is expected among species.  It should be mentioned that this conclusion is only tentative, since the lab has only examined a few host species and used only one gene, which can be misleading.  Furthermore, we have only examined one mite complex, and it is possible that others will show a different pattern of divergence.  But our preliminary studies do appear to suggest that morphological studies must be augmented with molecular studies if we are to truly understand the evolution and species delimitation in nasal mites.