I find invasive species fascinating. Either on their own steam or with a little help from us they have expanded their geographical distribution and, with time, they start forming part of the network of ecological links in each environment. Furthermore, - although they might have a negative impact on the economy and/or native species - they provide unique natural experiments that allow us to witness evolution in action and to investigate which evolutionary forces are be involved. One such invasive species is the Harlequin ladybird. This beautiful ladybird has gone from a native Central and Eastern Asian species to be considered a worldwide plague, via its use by us as an agent for the biological control of aphids. Despite the Harlequin having been used as such control in the U.S for decades, its range did not immediately expand and there seemed to be little room for concern. However, in 1988 the Harlequin started to become invasive and went to invade large areas in the U.S, Europe, South America and South Africa. In the U.K. the first sightings occurred in 2004 and it appears that by 2010 the ladybird had expanded into most available habitat.
A surprising fact of biological invasions is that they are successful, despite, stemming from a presumably small number of introduced individuals, what is termed a population bottleneck. Population bottlenecks are known to result in breeding between relatives and lead to strong inbreeding depression, a loss of fitness due to consanguineous matings. Why are then invasions successful? shouldn't they end in extinction? There are two ways out of this paradox. The first one, that there is no paradox at all. Invasive populations might actually not suffer a bottleneck, maybe because many individuals are introduced at once, or because repeated introductions replenish the population genetic diversity before the alien population becomes extinct and restores its fitness and invasibility. A second more interesting possibility stems from the small population surviving long enough to "purge" or remove from the population the alleles responsible for inbreeding depression as inbred individuals carrying two copies from these alleles die out. Purging can take place given certain combilations of population features such as population size, fertility and duration of the bottleneck. This possibility, although backed up by the theory and laboratory experiments with fruit flies, had not been documented to occur in the wild. In a recent paper, Facon and colleagues actually test (1) that bottlenecks actually occurred in the Harlequin ladybird and (2) that purging has happened in the invaded range. Their theoretical model, based on the genetic diversity of native and introduced populations, points to a most likely population bottleneck of 150 individuals lasting 20 generations; features that are compatible with purging to be able to occur, and fitting the large time gap between introduction and invasion. Then, they carried out experiments to compare the fitness (measuring generation time and lifetime number of offspring) of inbred and outbred invididuals from three invasive and three native populations.
The results were astounding (see Figure above): inbred invasive ladybirds have similar fitness than outbred invasive ones and they even match native outbred ladybirds. Native ladybirds, as expected, have strong inbreeding depression. This shows that invasive harlequin populations have got rid of the negative consecuences of mating between relatives, that is, purging has taken place. As in the frontline of an invasion, when population densities are low, and mating is more likely to happen between siblings, the evolution of the ladybirds during the lag period ended giving them their invasive "edge". It would be interesting to see if inbreeding depression accumulates again once harlequin populations become dense and settled in an area.
Facon B, Hufbauer RA, Tayeh A, Loiseau A, Lombaert E, Vitalis R, Guillemaud T, Lundgren JG, & Estoup A (2011). Inbreeding Depression Is Purged in the Invasive Insect Harmonia axyridis. Current biology : CB, 21 (5), 424-7 PMID: 21333536
Maderspacher, F. (2011). The benefits of bottlenecks Current Biology : CB, 21 (5)
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