Growth variation and life history trade-offs of the invasive plant Lythrum salicaria
Trade-offs are useful for understanding patterns of variation among life history traits because they can explain why fitness cannot be maximized for all traits. Among these traits is flowering phenology, which is of ecological importance in Lythrum salicaria (purple loosestrife), an invasive plant in North America that has spread across a wide latitudinal range. Selection on flowering time is affected by differences in season length among environments, with early flowering strongly favoured in the north. However, due to a positive correlation between size at flowering and time to flower, late-flowering genotypes are favoured in the south. I test the hypothesis that developmental constraints produce the trade-off between size at flowering and time to flowering by quantifying developmental variation of L. salicaria with the aid of logistic growth models. In these models, I analyzed the growth of 2278 individuals from 221 maternal half-sibs families that I raised in a common garden at the Queen's University Biological Station. These families were collected from a total of 20 populations of L. salicaria across a 10° latitudinal gradient of eastern North America. I use nonlinear mixed-effects models to estimate the contribution of family and population effects to overall variation. I found strong positive correlations between two traits measured in the common garden, time to flower and size at flower, and two components of the growth function, asymptotic size and time to inflection. I characterized individual variation among these parameters using a principal components analysis, identifying three axes of variation. The primary axis, capturing 53% of total variation, was associated with asymptotic size and time to inflection. These findings demonstrate the importance of heterochrony, a mode of developmental variation with an implicit trade-off, in the evolution of invasive populations of L. salicaria. By linking the divergence of development among populations with the divergence of flowering time, my study deepens our knowledge of the mechanistic basis of this variation.
URI for this recordhttp://hdl.handle.net/1974/28923
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