Abstract
Behavioural processes such as species recognition and mate attraction signals enforce and reinforce the reproductive isolation required for speciation. The earthworm Lumbricus rubellus in the UK is deeply differentiated into two major genetic lineages, ‘A’ and ‘B’. These are often sympatric at certain sites, but it is not known whether they are to some extent reproductively isolated. Behavioural tests were performed, in which individually genotyped worms were able to choose between soils previously worked either by genetically similar or dissimilar individuals (N = 45). We found that individuals (75%) were significantly (P < 0.05) more likely to orientate towards the soil conditioned by worms of their own lineage. Further testing involved a choice design based on filter papers wetted with water extracts of soils worked by a different genotype on each side (N = 18) or extracts from worked soil vs. un-worked control soil (N = 10). Again, earthworms orientated towards the extract from their kindred genotype (P < 0.05). These findings indicate that genotype-specific water-soluble chemicals are released by L. rubellus; furthermore, they are behaviour-modifying, and play a role in reproductive isolation between sympatric earthworm lineages of cryptic sibling species, through pre-copulatory assortative mate choice.
Highlights
An important question in the understanding of speciation is: what mechanisms are involved in the origin and maintenance of reproductive isolation between populations? Behavioural processes such as mate choice potentially play an important role in pre-copulatory reproductive isolation, leading to genetic divergence between the isolated lineages and, to speciation (Mendelson and Shaw, 2012)
Morphological stasis despite genetic divergence is commonly manifest in non-visually guided invertebrates that live in opaque media such as soil or turbid waters, where chemical signalling may play a more important role than morphology in sexual selection (Lee and Frost, 2002)
Given the known existence of cryptic, sympatric, earthworm lineages and the identification of sex pheromones in L. rubellus (Novo et al, 2013), our aim was to test the hypothesis that chemical cues play a role in reproductive isolation, through pre-copulatory assortative mate choice between cryptic species of earthworms
Summary
An important question in the understanding of speciation is: what mechanisms are involved in the origin and maintenance of reproductive isolation between populations? Behavioural processes such as mate choice potentially play an important role in pre-copulatory reproductive isolation, leading to genetic divergence between the isolated lineages and, to speciation (Mendelson and Shaw, 2012). Examples of signals that have contributed to behavioural isolation involve visual signals observed in butterflies (Wiernasz and Kingsolver, 1992), damselflies (Saetre et al, 1997), fish (Seehausen and Van Alphen, 1998) and frogs (Maan and Cummings, 2008), acoustic signals observed in insects, frogs (Gerhardt and Huber, 2002) and bats (Barlew and Jones, 1997), and chemical signals observed in moths (Linn and Roelofs, 1995), spiders (Trabalon et al, 1997), and flies (Coyne et al, 1994) Variation in these signals between lineages can result in pre-mating reproductive isolation, providing a behavioural mechanism driving speciation (Smadja and Butlin, 2009). Soil-dwelling cryptic species provide an ideal model for investigating veiled processes of behavioural isolation and mate recognition systems
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