Antagonistic pleiotropy and the evolution of alternate generations
The simple (but elegant) series of experiments performed by McHaffie et al. (see pp. 491-500 in this issue) - involving studies of segregation from field-collected sporophytes, sporophytic progeny of selfed gametophytes, and the morphology of sporophytes from reciprocal crosses - conclusively demonstrates the genetic basis of the intervarietal morphological differences. These differences are further conditioned by one gene and two alleles, with the 'distentifolium allele' (AD) dominant over the 'flexile allele' (AF). Did the authors also achieve their goal of determining those factors that might help explain how flexile is maintained within populations? The issue is complex. Gametophytes of the flexile type seem to have a consistent growth advantage, and it might be that this translates into a reproductive advantage. In the sporophyte generation, pleiotropic effects of the AF allele appear to confer an advantage in terms of frond number and fertility to the flexile form at high nutrient concentrations, whereas at the same high nutrient levels the AD (distentifolium) allele is associated with larger fronds. From this, the authors hypothesize 'antagonistic pleiotropy' for the maintenance of the AF allele in Scottish populations of A. distentifolium. Whether or not this is correct, they have provided new information about this fern that has implications for both conservation and basic evolutionary biology.
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