Unravelling the genetic basis and evolution of heterostyly

Primary author: Paige Henning
Co-author(s): Joel Shore; Andrew McCubbin
Faculty sponsor: Andrew McCubbin

Primary college/unit: Arts and Sciences
Campus: Pullman


Heterostyly has fascinated botanists for centuries. It is a plant breeding system analogous to separate sexes, combining morphological and physiological reproductive traits to promote outbreeding, but individuals are hermaphrodite. It provides an interesting example of convergent evolution having evolved independently many times. In heterostylous species, individuals produce flowers of one of two morphs, the sex organs (female stigmas and styles, and male pollen producing anthers) positioned reciprocally between morphs. This promotes between- and reduces within-morph pollination. Biochemical compatibility also differs, preventing successful within morph pollination. Genetically, heterostyly is encoded by a single group of linked genes termed the S-locus, plants with short styles historically being viewed as heterozygous, and those with long styles homozygous recessive. We previously mapped and sequenced the S-locus of Turnera subulata. The S-locus was found not to be allelic, but composed of 3 genes specific to the dominant allele (i.e. hemizygous). Only one, TsBAHD, is expressed in female tissues and hence controls female morphology and physiology. The other two, TsYUC6 and TsSPH, are expressed in anthers (male), and TsSPH also in filaments. Homology based hypotheses were proposed for the functions of these genes. Here we use RNAseq data and bioinformatic approaches to empirically test these hypotheses. Support was found for TsBAHD controlling female characters by degrading brassinosteroids and for TsYUC6 controlling stamen characters by synthesizing auxin. Unexpectedly results also suggested that these changes in single hormone levels interface with (a) major signaling hub(s), leading us to speculate commonality in mechanism for the convergent evolutions of heterostyly.