title: Understanding genetic and phenotypic diversity in the speciation of Petunia : evolution of quantitative and qualitative traits
Creators: name: Binaghi, Marta
Publisher: name: Universität Bern
Year: 2022
Subjects: subject: 580 Plants (Botany)

description: The diversity of flowering plants is connected with the recruitment of pollinators in the reproductive process. Some species of plants have specialised floral phenotypes that attract and reward one or few pollinators, and that allow efficient pollen exchange. The pollinator specificity of flowers can constitute a barrier to reproduction, by isolating plants pollinated by different animals. The group of traits (termed “pollination syndrome”) that defines which pollinator will transfer pollen efficiently can thus define species isolation. In this manuscript I focus on the genetic basis of the changes in pollination syndrome traits in the Petunia genus.

P. axillaris and P. exserta have different pollinators, and their pollination syndrome traits differ in UV and visible colour, in scent production and in floral morphology. We investigated flower colour and morphology in individuals at two contact zones where the species live in sympatry and produce hybrid offspring. We used a genome-wide variant dataset to describe the population genetics of the individuals, and combined genotype information with phenotype data to estimate the genetic architecture of the traits and identify associated loci. The traits studied reveal different genetic architectures. Visible colour is associated with a polygenic basis, while UV colour with an oligogenic basis including less than 10 genes. Both the morphological traits (pistil and tube length) are predicted to have an oligogenic basis. We thus highlight that the classic association of discrete traits with a simple genetic basis, and continuous traits with a complex genetic basis is not always valid. The selection scan on these individuals identifies two broad regions experiencing positive selection. The regions do not include the speciation gene MYB-FL, responsible for the UV colour difference between the species. We suggest that this gene and the region associated with it are not under selection at the contact zones, maybe for a loss of importance of the trait in these populations. We then suggest that the regions positively selected may be the result of selection pressure acting on the morphology phenotypes, or may be due to incompatibilities developed during the divergence of the species.

Scent production has changed several times during Petunia speciation. In particular, the shift from the bee pollination of P. inflata to the hawkmoth pollination of P. axillaris included an increase in the amount and number of volatiles produced. The biosynthesis of volatiles and the genetic changes that underlie scent production have been characterised in these species and have a simple genetic basis. On the other hand, the emission route of these volatiles has only been described in the research cultivar P. hybrida cv Mitchell. Scent production underwent important changes during Petunia evolution. We thus ask if these changes were accompanied by the adaptation of the scent emission system to sustain the new and increased production. We investigated the sequence conservation of the P. hybrida cv Mitchell transporter ABCG1 in the wild Petunia species, and we measured its expression in the strongly scented P. axillaris. P. axillaris and P. hybrida cv Mitchell share phenotypic similarities in their scent profiles as well as in other floral traits. P. hybrida cv Mitchell is derived from a series of crosses involving P. axillaris as a recurrent parent and a commercial variety descended from a species of the bee-pollinated short-tube clade (such as P. inflata). Consistently with its origin, the transcriptome of P. hybrida cv Mitchell is mostly constituted of P. axillaris transcript alleles. We thus hypothesised that the scent transporter ABCG1 function is conserved in P. axillaris. Unexpectedly we found that the sequence of the protein of P. hybrida is most similar to a species of the short-tube clade, suggesting that it was inherited from a modestly scented Petunia. Even more surprisingly, the ABCG1 transporter is not expressed in the scented P. axillaris. We thus identify other ABCG subfamily members that could effect scent emission in this wild Petunia by analysing their expression profiles. We suggest that another gene may be responsible for scent emission in P. axillaris, or alternatively that P. axillaris does not need a transporter to emit scent. While the scent biosynthetic pathway is generally conserved between P. hybrida cv Mitchell and P. axillaris, scent emission took a different route.