Historical Factors and Anisoplethic Population Structure in Tristylous Pontederia cordata: A Reassessment

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Abstract
Theoretical models of floral-morph frequencies in tristylous species predict a single equilibrium with all three morphs represented in equal proportions (isoplethy). North American populations of Pontederia cordata exhibit considerable heterogeneity of morph frequencies between populations, with the short-styled morph often in excess of isoplethic expectations and the long-styled morph commonly underrepresented. In a previous study, it was proposed that anisoplethic population structure in P cordata is the result of differential male fertility, owing to genetic differences in pollen production among the morphs. In this study, the influence of historical factors on morph frequencies prior to equilibrium was investigated using a deterministic computer model. Nonequilibrium frequencies are strongly influenced by the genotypes of founding individuals, and, because tristyly is under the control of two diallelic loci, phenotypic equilibrium is approached asymptotically. The model indicates that in nonequilibrium populations the short-styled morph will be in excess and the long-styled morph will be underrepresented. This suggests that historical factors play an important role in determining population structure in P cordata. Several features of the population ecology of the species lend support to this interpretation. Historical factors should be taken into account when interpreting data from population surveys of morph frequencies in tristylous species and of other genetic polymorphisms not under single-locus control.

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