Modularity and selection of nectar traits in the evolution of the selfing syndrome in Ipomoea lacunosa (Convolvulaceae)

Abstract

Although the evolution of the selfing syndrome often involves reductions in floral size, pollen, and nectar, few studies of selfing syndrome divergence have examined nectar. We investigate whether nectar traits have evolved independently of other floral size traits in the selfing syndrome, whether nectar traits diverged due to drift or selection, and the extent to which quantitative trait locus (QTL) analyses predict genetic correlations. We use F5 recombinant inbred lines (RILs) generated from a cross between Ipomoea cordatotriloba and I. lacunosa. We calculate genetic correlations to identify evolutionary modules, test whether traits have been under selection, identify QTLs, and perform correlation analyses to evaluate how well QTL properties reflect the genetic correlations. Nectar and floral size traits form separate genetic clusters. Directional selection has acted to reduce nectar traits in the selfing I. lacunosa. Calculations from QTL properties are consistent with observed genetic correlations. Floral trait divergence during mating system syndrome evolution reflects independent evolution of at least two evolutionary modules: nectar and floral size traits. This independence implies that adaptive change in these modules requires direct selection on both floral size and nectar traits. Our study also supports the expected mechanistic link between QTL properties and genetic correlations.

How broad is the selfing syndrome? Insights from convergent evolution of gene expression across species and tissues in the Capsella genus

“…Pollen is critical for plant reproductive success and is predicted to be under strong selective force in outcrossing species as a result of the pervasive male–male gamete competition, resulting in the outcrossing species being prone to generating a larger number of pollen grains (Harvey & May, 1989). By contrast, selfing species generally show a lower number of pollen grains, because fertilization is easier to complete in a selfing species than in its outcrossing relatives (Willis, 1999; Cruden, 2000; Shimizu & Tsuchimatsu, 2015; Liao et al ., 2022). Because the selfing species does not need to attract pollinators, some rewarding products for pollinators, such as scents, nectars and feeding anthers, that are costly for the plant to produce, will no longer be needed and thus will be under relaxed selection (Smith, 2016; Wessinger & Hileman, 2016; Liao et al ., 2022).…”

“…By contrast, selfing species generally show a lower number of pollen grains, because fertilization is easier to complete in a selfing species than in its outcrossing relatives (Willis, 1999; Cruden, 2000; Shimizu & Tsuchimatsu, 2015; Liao et al ., 2022). Because the selfing species does not need to attract pollinators, some rewarding products for pollinators, such as scents, nectars and feeding anthers, that are costly for the plant to produce, will no longer be needed and thus will be under relaxed selection (Smith, 2016; Wessinger & Hileman, 2016; Liao et al ., 2022). Selfing species also exhibit an early mature time of pollen development (de Jong et al ., 1993) and a lower amount of fruit set (Bellusci et al ., 2009; Jacquemyn & Brys, 2015).…”

Scott Armbruster

“…I am impressed by the detailed insights into the effects of floral morphology on pollination precision revealed by Stewart et al (2022). Another interesting topic addressed in a very original way is the investigation of the significance and basis of intra-floral modularity in Ipomoea, as revealed in an impressive study by Liao et al (2022).…”

“…Another interesting topic addressed in a very original way is the investigation of the significance and basis of intra‐floral modularity in Ipomoea , as revealed in an impressive study by Liao et al . (2022). At a larger scale, valuable insights into floral evolvability come from an exciting integrative study of plasticity and deep‐phylogenetic patterns of phenotypic disparity in Brassicaceae by Gómez et al .…”