QUANTITATIVE GENETICS OF LIFE-HISTORY AND MORPHOLOGY IN A RARE PLANT,SENECIO-INTEGRIFOLIUS

A paternal half-sib analysis was performed to estimate the narrow-sens e heritability for 35 morphological and life-history characters in two populations of Senecio integrifolius, a rare and threatened plant in Sweden. The two populations differed greatly in size, the small one co nsisting of separate patches and the large one having a more or less c ontinuous distribution. In addition to significant population differen ces in the mean of 26 characters, we found slightly higher heritabilit ies in the small and patchy population (average 0.49) than in the larg e and continuous population (average 0.43). Overall, the small populat ion displayed significant additive genetic variation for a larger numb er of characters than the large population (19 vs. 14). Paternal (gene tic) effects were consistent in two different environments; the rankin g of sire means was similar despite a drastic reduction in light inten sity, and few characters had a significant sire x environment interact ion. Genetic correlations were also found between different characters measured in the same environment, although most of these were in the 'favourable' direction. Population differences in genetic parameters m ay reflect contrasting selection regimens at the two sites, although g enetic drift may also be important. That more genetic variability was detected in the small population where plants occur in small and isola ted patches indicates that the spatial structure has great effects on the retention of heritable variation, and that demographic and environ mental factors are more important in the long-term survival of S. inte grifolius. This is the first study that relates quantitative genetic v ariation to population size in a rare and threatened plant.