Academic journal article Genetics

Population Genetics and a Study of Speciation Using Next-Generation Sequencing: An Educational Primer for Use with "Patterns of Transcriptome Divergence in the Male Accessory Gland of Two Closely Related Species of Field Crickets"

Academic journal article Genetics

Population Genetics and a Study of Speciation Using Next-Generation Sequencing: An Educational Primer for Use with "Patterns of Transcriptome Divergence in the Male Accessory Gland of Two Closely Related Species of Field Crickets"

Article excerpt

SUMMARY Understanding evidence for the genetic basis of reproductive isolation is imperative for supporting students' understanding of mechanisms of speciation in courses such as Genetics and Evolutionary Biology. An article by Andrés et al. in the February 2013 issue of GENETICS illustrates how advances in DNA sequencing are accelerating studies of population genetics in species with limited genetic and genomic resources. Andrés et al. use the latest sequencing technologies to systematically identify and characterize sites in the DNA that vary within, and have diverged between, species to explore speciation in crickets. This primer, coupled with that article, will help instructors introduce and reinforce important concepts in genetics and evolution while simultaneously introducing modern methodology in the undergraduate classroom.

Background

Some of the fastest evolving proteins in insect genomes are transferred from males to females in their seminal fluid (Findlay and Swanson 2010). Amino acid sequence divergence in these proteins can contribute to speciation by creating incompatibilities between sperm and eggs when two distantly related genotypes mate. Based on these observations, it has been proposed that divergent seminal fluid proteins might contribute to the early stages of speciation in closely related species of field crickets Andrés et al. (2006). Andrés et al. (2013) explore this hypothesis by characterizing DNA sequence variation in the coding regions of proteins expressed in accessory glands where seminal proteins are found.

Study system: Gryllus firmus and Gryllus pennsylvanicus

The senior author of the Andrés et al. (2013) study, Richard G. Harrison, began investigating speciation in crickets in the mid-1970s using protein electrophoresis techniques. Since then, he and his colleagues have examined properties of cricket mitochondrial DNA (Harrison et al. 1985; Rand and Harrison 1986, 1989), studied hybrid zones where different species of crickets meet (Willett et al. 1997; Ross and Harrison 2002; Maroja et al. 2009), and investigated whether the bacterial parasite Wolbachia (Mandel et al. 2001; Maroja et al. 2008) and divergent reproductive proteins (Andrés et al. 2006, 2008) contribute to speciation in crickets. Most recently, their studies have focused on G. firmus and G. pennsylvanicus, a pair of closely related species of field cricket found in the eastern region of North America. G. firmus is found on sandy soils along the east coast, and G. pennsylvanicus is found on firmer inland loam soils. Hybrids resulting from mating between these two species are found along the eastern edge of the Appalachian Mountains (Figure 1). When G. pennsylvanicus females mate with G. firmus males, viable and fertile offspring are produced; however, when G. pennsylvanicus males mate with G. firmus females, no hybrid offspring are observed (Harrison 1985).

Using sequence variation to identify genes contributing to speciation

G. firmus and G. pennsylvanicus are thought to have become different species via allopatric speciation (Willett et al. 1997). This is a mode of speciation in which populations of the same species are geographically isolated from each other and thereby evolve genetic differences over time that reduce their ability to produce fully fertile offspring when their ranges overlap. When the isolation initially occurs, the two populations share alleles throughout the genome. Over generations, mutation, drift, and selection cause genetic differences to accumulate between the two populations. Most of these genetic differences will have no effect on morphology, physiology, or the ability of individuals from one population to interbreed with individuals from the other population-but some will affect one or more of these traits.

When hybridization between two species occurs and fertile offspring are produced, alleles are moved between species. This mixing of genetic information, or gene flow, causes introgression that reduces the genetic divergence between species. …

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