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Patterns of style polymorphism in the South African genus Nivenia Ventenat (Iridaceae)

Heterostylous ("reciprocal herkogamous") plants have two or three discrete morphs that differ in their sex organ position within populations. The flowers of the morphs are complementary, so the stigmas of one morph are placed at the same level than the anthers of the other, and the other way round.

Fig.1.- Representation of the two floral morphs in Nivenia, with the variables measured for each flower

This polymorphism is widely distributed among the angiosperms, but detailed studies are limited to few taxonomic groups, and evolutionary meaningful variations of the heterostylous syndrome have been reported. A thorough exploration of groups where heterostyly has been reported should offer new opportunities to further testing the evolutionary hypotheses explaining heterostyly. Here we explore the traits defining heterostyly in several species of Nivenia, the only genus of Iridiaceae where heterostyly has been reported.

In this work a detailed morphometric analysis of the flower sexual whorls and some traits considered as ancillary are supplied to determine for each population the morph ratio and the degree of reciprocity between morphs. We also estimate the rates of assortative (within morph) versus disassortative (between morphs) pollen transfer by analyzing pollen loads on stigmas. The association between floral phenotypic integration (i.e. correlation between floral characters) and the reciprocity between morphs was estimated; both characteristics have been quoted as dependent on the accuracy of the fit between pollinators and flowers and therefore related to the efficiency of pollen transfer.

Fig.2.- N.inaequalis "short" morph flower

We found different degrees of reciprocity within Nivenia. Effective disassortative mating appears to be common, since all dimorphic populations show equal morph-ratios (isoplethy), and the pollen placed on the stigmas of each morph is likely coming from the other (complementary) morph.

Fig.3.- Between-morph relationship in pollen grain size, both for the grains in the anthers (i.e. before dispersion), and on the stigma surface (after pollination)

The most reciprocal populations of the heterostylous species have also the highest values of phenotypical integration.

Therefore, stigma height dimorphism, as opposed to heterostyly, is proven for the first time in Nivenia. The presence of different degrees of the polymorphism within the genus is consistent with hypotheses of the evolution of heterostyly that consider pollinators as the leading evolutionary force. Those hypotheses are also supported by the relationship between floral integration and reciprocity.

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