A population acquires the status of a new species when it no longer crosses with others. This reproductive isolation generally results from the geographical separation of populations which become genetically incompatible. It may also arise because of ecological divergence linked to a specialisation of different habitats or resources that reduces the opportunities for reproduction between populations. Although this latter speciation model is theoretically well established, empirical confirmations remain restricted to a few species, such as the cichlids of Lake Victoria and Darwin's finches of the Galapagos Islands.
In insects subservient to plants, this type of speciation pathway without geographical isolation may pass through the "host race" stage of populations that are partially isolated from a reproductive point of view, specialised on different host plants and coexisting in the same geographical region. To test this hypothesis, the Joint Research Unit for the Biology of Organisms and Population Biology applied to Plant Protection (BiO3P) used the pea aphid Acyrthosiphon pisum as its study model: although this is currently designated as a single species, A. pisum in fact appears to constitute a group of populations displaying different preferences and adaptations to a variety of species in the legume family.
A complex of host races and developing species
The analysis concerned 1700 aphids collected from 19 species of wild-type (sweet clover, broom, vetch, etc.) and cultivated (pea, alfafa, clover, etc.) legume species at different sites in Europe. Their genetic characterisation was determined using DNA markers (microsatellites), so that it was possible to assign the aphids individually to homogeneous genetic groups.
The classification based on this genetic characterisation alone was able to identify eleven, clearly distinct, genetic groups associated with different host plants but not with the region of origin of the individuals. This host specialisation of the eleven groups was confirmed by laboratory tests: the lines arising from each genetic group performed better (in terms of survival and growth) on the plant with which they were associated in the field.
This coincidence between the genetic type of an individual and the plant from which it was harvested was observed in more than 90% of cases. The association was thus strong but not total; the existence of a small percentage of "migrant" individuals (present on a plant that did not correspond to their genetic type) suggested the possibility of hybridization between biotypes. First-generation hybrids between two types were thus searched for in the sample. The proportion of hybrids, which reflected the degree of reproductive isolation of genetic groups, was null for three of them, which could thus be considered as well-formed species; the eight others, which displayed variable proportions of hybrids (with a maximum of 9%) constituted host races with gene flows subsisting between them.
Rapid and recent diversification linked to ecological factors
A supplementary study, based on an analysis of divergence rates between different pea aphid biotypes using rapidly evolving DNA sequences, made it possible to date this adaptive radiation to only 10,000 years. This diversification was thus extremely rapid and coincided with the global warming that followed the last Ice Age and with early forms of agriculture. It is probable that domestication, or changes to the distribution areas of certain host plants for pea aphids, may have caused abrupt differentiation that led to the formation of this biotype complex.
These studies thus demonstrate the existence of a complex of numerous and genetically differentiated entities, each specialised on one or a few legume species. These entities form a remarkable continuum of speciation that links partially isolated "host races" with well-formed species, reflecting a progressive and rapid evolution towards reproductive isolation through ecological speciation. These findings confirm Darwin's idea that species divergence is a gradual process guided by natural selection. They also emphasise the importance of ecological and anthropogenic factors to the dynamics of biodiversity.
References:
A continuum of genetic divergence from sympatric host races to species in the pea aphid complex
PNAS 2009, 106: 7495-7500
Jean Peccoud 1, Anthony Ollivier 1, Manuel Plantegenest 2, Jean-Christophe Simon 1
1. UMR 1099 BiO3P (Biologie des Organismes et des Populations appliquée à la Protection des Plantes), INRA, Domaine de la Motte, 35653 Le Rheu, France
2. UMR 1099 BiO3P, Agrocampus Ouest, 65 rue de Saint-Brieuc, 35042 Rennes, France
Post-Pleistocene radiation of the pea aphid complex revealed by rapidly evolving endosymbionts
PNAS 2009, 106: 16315-16320
Jean Peccoud 1, Jean-Christophe Simon 1, Heather McLaughlin 2, Nancy Moran 2
1. UMR 1099 BiO3P (Biologie des Organismes et des Populations appliquée à la Protection des Plantes), INRA, Domaine de la Motte, 35653 Le Rheu, France
2. Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85718
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