Academic journal article Genetics

Molecular Differentiation at Nuclear Loci in French Host Races of the European Corn Borer (Ostrinia Nubilalis)

Academic journal article Genetics

Molecular Differentiation at Nuclear Loci in French Host Races of the European Corn Borer (Ostrinia Nubilalis)

Article excerpt

ABSTRACT

French populations of the European corn borer consist of two sympatric and genetically differentiated host races. As such, they are well suited to study processes that could be involved in sympatric speciation, but the initial conditions of host-race divergence need to be elucidated. Gene genealogies can provide insight into the processes involved in speciation. We used DNA sequences of four nuclear genes to (1) document the genetic structure of the two French host races previously delineated with allozyme markers, (2) find genes directly or indirectly involved in reproductive isolation between host races, and (3) estimate the time since divergence of the two taxa and see whether this estimate is compatible with this divergence being the result of a host shift onto maize after its introduction into Europe ~500 years ago. Gene genealogies revealed extensive shared polymorphism, but confirmed the previously observed genetic differentiation between the two host races. Significant departures from the predictions of neutral molecular evolution models were detected at three loci but were apparently unrelated to reproductive isolation between host races. Estimates of time since divergence between French host races varied from ~75,000 to ~150,000 years, suggesting that the two taxa diverged recently but probably long before the introduction of maize into Europe.

THE European corn borer (ECB), Ostrinia nubilalis Hübner (Lepidoptera: Crambidae), exists as a number of sympatric, genetically differentiated but partially interfertile taxa across its geographical range in northern Eurasia and northern America (HUDON et al. 1989). As such, it is a well-suited biological model to study processes that could be involved in the early steps of speciation, especially sympatric and ecological speciation (SCHLUTER 2001; VIA 2001; BERLOCHER and FEDER 2002; GAVRILETS 2003; RUNDLE and NOSIL 2005).

The ECB's area of origin is thought to be central Asia (HUDON et al. 1989). Its larvae feed on a large number of wild and cultivated host plants (CAFFREY and WORTHLEY 1927; HODGSON 1928; PONSARD et al. 2004), but it is commonly found on maize (Zea mays L.). ECB populations collected as larvae on maize stalks in certain locations in Europe (PEÑA et al. 1988) and in North America (KLUN 1975; KOCHANSKY et al. 1975; GLOVER et al. 1990) are polymorphic with respect to sex pheromone communication. On the basis of this polymorphism, two pheromonal races can be distinguished: the Z-race using a 97:3 blend of Z:E isomers of the 11- tetradecenyl acetate (11-14:OAc), and the E-race using blends of Z11-14:OAc and E11-14:OAc in a 1:99 to 4:96 ratio (KLUN 1975). The percentage of hybrid moths is lower than expected under random mating between E- and Z-race moths, suggesting partial reproductive isolation between the two pheromonal races (GLOVER et al. 1990).

In contrast to other locations in Europe and North America, ECB pheromonal races in France can be distinguished according to the host plant(s) they use (BOURGUET et al. 2000; MARTEL et al. 2003). Indeed, in France, but not in the Balkans, Italy, and North America, maize is infested exclusively by moths using the Z sex pheromone blend (PÉLOZUELO et al. 2004), whereas hop (Humulus lupulus L.) and mugwort (Artemisia vulgaris L.) are infested exclusively by moths using the E pheromone blend (THOMASET al. 2003; PÉLOZUELO et al. 2004). These host races have thus been referred to as the "maize race" and the "mugwort-hop race," respectively. The percentage of hybrids between the two host races is lower than those observed in the United States between the two pheromonal races (MALAUSA et al. 2005), suggesting that the pheromone difference is probably not the only factor serving to isolate the maize race from the mugwort-hop race.

Given that maize was introduced into Eurasia only ~500 years ago and has become a major host plant of many ECB populations worldwide, it is reasonable to assume that populations of the maize race have adapted recently to this host plant (THOMAS et al. …

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