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Sexual systems are highly variable in flowering plants and an important contributor to floral diversity. The ranunculid genus  Thalictrum  is especially well-suited to study evolutionary transitions in sexual systems. Homeotic transformation of sexual organs (stamens and carpels) is a plausible mechanism for the transition from hermaphroditic to unisexual flowers in this lineage because flowers of dioecious species develop unisexually from inception. The single-copy gene  PISTILLATA  ( PI ) constitutes a likely candidate for rapid switches between stamen and carpel identity. Here, we first characterized the expression pattern of all B class genes in the dioecious species  T. dioicum . As expected, all B class orthologs are expressed in stamens from the earliest stages. Certain  AP3  lineages were also expressed late in sepal development. We then tested whether orthologs of  PI  could potentially control sexual system transitions in  Thalictrum , by knocking-down their expression in  T. dioicum  and the hermaphroditic species  T. thalictroides . In  T. dioicum , we found that  ThdPI-1/2  silencing caused stamen primordia to develop into carpels, resulting in male to female flower conversions. In  T. thalictroides , we found that  ThtPI  silencing caused stamen primordia to develop into supernumerary carpels, resulting in hermaphroditic to female flower conversions. These phenotypes illustrate the ability for homeotic mutations to bring about sudden and potentially adaptive changes by altering the function of a single gene. We propose a two-step evolutionary model where transitions from hermaphroditic to unisexual plants in  Thalictrum  result from two independent mutations at a B class gene locus. Our  PI  knockdown experiments in  T. thalictroides  recapitulate the second step in this model: the evolution of female plants as a result of a loss-of-function mutation in a B class gene.

Functional recapitulation of transitions in sexual systems by homeosis during the evolution of dioecy in Thalictrum