Inferences about the origin of a field cricket hybrid zone from a mitochondrial DNA phylogeny

Abstract

Two closely related eastern North American field crickets, Giyllus firmus and G. pennsylvanicus, hybridize along a zone that extends from Connecticut and the Hudson River Valley, south along the eastern front of the Appalachian Mountains to at least Virginia. Here we use mitochondrial DNA (mtDNA) sequences to construct a population phylogeny for this pair of hybridizing cricket species. Using a phylogenetic approach, we attempt to discriminate between alternative population histories (secondary contact vs. primary intergradation) leading to formation of the hybrid zone. A strict consensus tree, based on >1600 bp of the COl-COIl region of the mtDNA genome, reveals four exclusive groups, which correspond to regional groupings of conspecific crickets. Surprisingly, the mtDNA sequence data do not reveal any synapomorphies for either G. pennsylvanicus or G. firmus. However, the mtDNA data do reveal a clear north-south split within each of the cricket species, a pattern not seen for morphological or other molecular characters. The biogeographical history of the north-south divergence events remains a puzzle. Observed gene genealogies support a model of secondary contact for the southern part of the hybrid zone. Sequence divergence data argue that lineages currently found in New York and New England were already distinct when this region became habitable following the most recent glaciation.

A Fine-Scale Spatial Analysis of the Mosaic Hybrid Zone Between Gryllus Firmus and Gryllus Pennsylvanicus

“…EcoRV cuts the PCR product once in G. pennsylvanicus from Connecticut, producing fragments of 1100 bp and 900 bp from the original 2000-bp PCR product. For G. firmus, EcoRV cuts the PCR product twice, producing fragments approximately 900 bp, 800 bp, and 300 bp in size (for DNA sequence see Willett et al 1997). This restriction site difference is diagnostic for populations of G. pennsylvanicus and G. firmus outside of the hybrid zone in the Northeast (Harrison et al 1987;Harrison and Rand 1989).…”

“…For mtDNA, total DNA was extracted from femoral muscle using Qiagen DNeasy tissue kits (Qiagen, Inc., Valencia, CA). An approximately 2-kb segment of mtDNA spanning the cytochrome oxydase I and II (COI and COII) region was amplified using polymerase chain reaction (PCR) with the Harrison lab primers Ron (5Ј-GCATCACCTGATAT-AGCATTCCC-3Ј) and Eva (5Ј-GAGACCATTACTTGCTT-TCAGTCATCT-3Ј; Simon et al 1994;Willett et al 1997). For 10-l PCR reactions, 1 l of a 1/1000 dilution of total DNA was added to a reaction mixture (1 ϫ PCR buffer [Gibco-BRL, Rockville, MD], 3 mM MgCl 2 , 0.2 mM of each primer, 0.05 units TAQ [Gibco-BRL], and 0.2 mM dNTPs).…”

Genetics and the origin of species: the continuing synthesis a symposium in honor of Richard G. Harrison

“…Over the course of the next two decades, he and his students and colleagues would use studies of mtDNA to assess evolutionary relationships, population genetic structure, and species delimitations of a variety of different animal groups (Normark et al 1991; Brown et al 1994; Nurnberger and Harrison 1995; Harrison et al 2003; Hatch et al 2006; Maroja et al 2007; Nydam and Harrison 2007). They also used studies of mtDNA to test the avian constraint hypothesis (Stanley and Harrison 1999), models for the origin of hybrid zones (Willett et al 1997), the impact of past glaciations on population differentiation and speciation (Sperling and Harrison 1994), the origin of the genus Schistocerca in North America (Lovejoy et al 2006), hypotheses of Pleistocene speciation in copepods (Thum and Harrison 2009), and the origin of invasive beetle populations (Carter et al 2009, 2010). …”

Morphological differentiation despite gene flow in an endangered grasshopper

“…Historically, one of the most informative animal groups in this field of study have been Orthoptera and in particular grasshoppers [14,31-35]. Here we report on flightless New Zealand short-horned grasshoppers (Orthoptera: Acrididae).…”