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A chromosome marker for natural and anthropogenic change in the predatory marine gastropod Nucella lapillus (L.)

Final Activity Report Summary - CHROMOVAR (A chromosome marker for natural and anthropogenic change in the predatory marine gastropod Nucella lapillus (L.))

Changes in chromosome number based on centromere fusion or fission (Robertsonian polymorphism) in the common dog-whelk Nucella lapillus results in striking chromosome number differences at different localities on the French and English Channel coasts. This simple numerical variation has been accompanied by chromosomal inversions and other structural re-arrangements that have the potential to act as post-mating barriers to limit cross-breeding between chromosomally dissimilar forms. Hence, whilst the European dog-whelk appears to be a single species across its whole range, from Norway to Portugal, it may hold within its cells the secret of how different species may arise without geographic isolation through subtle changes in chromosome structure and number.

Focusing on dog-whelks with diploid chromosome numbers of 2n = 26 and 2n = 36, we have shown using laboratory breeding experiments that the number of offspring hatching from crosses between the two types is reduced compared to crosses within each pure parental form. While reproductive output was lowest in hybrid crosses involving female 2n=36 individuals (i.e. crossed with a male 2n = 26), the reciprocal cross involving a female 2n=26 form performed as well in terms of reproductive output as those where both parents were 2n=26. This strongly supports the existence of a partial barrier to cross breeding between these chromosomally distinct forms, at least in one direction. Whether this barrier is immunological (sperm recognition), allelic (genes) or chromosomal (meiotic instability) remains to be identified, but this points to the subtle barriers to cross breeding that may eventually result in local speciation and/or adaptation.

In a world threatened by accelerated climate change, it is essential that we understand more about those intrinsic mechanisms that act as barriers/corridors affecting the rates of genetic exchange within and between populations. Our research findings suggest that the humble dog-whelk, with its intriguing chromosome polymorphism, may hold the key to unravelling the mysteries of evolution on a micro-scale, which may offer important insights into how populations of marine organisms adapt and survive under conditions of environmental change.