Plasticity and the genetics of reproductive behaviour in the monocarpic perennial, Lobelia inflata (Indian tobacco)

The timing of reproduction is an important life-history variable, especiall y for organisms that die following a single reproductive episode, such as t he monocarp Lobelia inflata. The propensity to initiate flowering (to bolt) under a given set of conditions is expected to be shaped by natural select ion acting on the norms of reaction for bolting behaviour over, for example , changing photoperiods. We study the genetic basis of bolting and of the p lasticity of bolting using three continuously changing photoperiod regimes over two generations in a growth chamber experiment. Multiple genotypes fro m three populations are tested under three different photoperiod treatments mimicking early, mid, and late 'summer' during both generations. The frequ ency of bolting ranges from 88% under long days to 1% under short days. The overall heritability (h(2)) of bolting is found to be high, and increases later in the flowering season. Genetic variance for bolting is explained by genetic variance for threshold size itself, rather than for capacity to at tain a fixed threshold size: genotypes that bolt most readily tend to be th ose that bolt at a smaller rosette size. No significant heritability of the plasticity of bolting behaviour is detected. Similarly to within populatio ns, variation at the among-population level exists for bolting behaviour. T here is no evidence for genetic population differentiation with respect to plasticity for bolting: although plasticity differs among populations withi n a generation, this population effect is not consistent between the two ge nerations of the experiment.