Phenotypic plasticity and genetic variability in annual <i>Zizania</i> spp. along a latitudinal gradient

Abstract

Bulk seed collections were made from six populations of Zizania spp. (two Z. palustris and four Z. aquatica) along the Atlantic seaboard from northern New Brunswick, Canada, to southern Georgia, U.S.A. Ten plants per population were cultivated in each of two greenhouse chambers differing in temperature, and 20 – 80 seedlings from each population were assayed for 14 isozyme loci. Mean values for a suite of phenological and morphometric traits showed clinal variation in association with the latitudinal gradient. Magnitudes and patterns of plasticity for the same traits did not show clinal variation in response to varying temperature regimes but were different in the two species. Plants from Z. palustris populations responded to elevated temperatures by delaying flowering, increasing stem size, and decreasing proportional reproductive effort; Z. aquatica populations were generally much less sensitive to the temperature differential, but where responses were observed they were typically opposite to those described for Z. palustris. The species were also differentiated by allelic composition of the populations. There was no systematic relationship between degree of heterozygosity at isozyme loci and magnitudes of plasticity for morphometric traits. There was no evidence for the coordinated divergence of Zizania populations for trait means and magnitudes and patterns of plasticity. It is suggested that ecological differences, possibly characteristic differences in intensity and consistency of competitive pressure, may account for the contrasting patterns of plasticity in Z. palustris and Z. aquatica. Key words: Zizania, phenotypic plasticity, latitude, genetic variability.