Investigating hybridization among high altitude Rhodohypoxis species in the Drakensberg Alpine Centre

Abstract

Putative hybridization between the alpine Rhodohypoxis taxa (R. baurii var. confecta, R. deflexa, R. rubella, R. incompta, and R. thodiana) as well as Hypoxis parvula var. parvula has been documented in the wild – suggesting that there are few genetic barriers between these taxa that often grow in sympatry. This study aimed to assess patterns of hybridization among the alpine Rhodohypoxis species and between them and H. parvula var. parvula at three different natural study sites and to contribute new data that might help delineate/solidify genus and species boundaries in this group using morphometric analysis, seed surface micromorphological comparisons, microsatellite data, and controlled greenhouse crosses. It was found that although all the methodologies were congruent in their ability to delineate the Rhodohypoxis species from one another and from H. parvula var. parvula (thereby corroborating their current circumscription), substantial admixture was detected at each of the three study sites – and admixed individuals were largely morphologically cryptic. Furthermore, morphometric analysis and seed surface micromorphology could not distinguish between R. thodiana and putative hybrid taxa – suggesting it may be of hybrid origin. The present study also reports the complete self-incompatibility of polyploid H. parvula var. parvula, the facultative self-compatibility of diploid R. baurii var. confecta, the successful crossbreeding of these two taxa from Sentinel Peak with a strong asymmetrical barrier to gene exchange, and a mechanism involving double-reduced polyploid pollen facilitating this interploidy cross. Low germination rate of the resulting hybrid seed is consistent with a decrease in F1 viability. These findings, coupled with the low incidence of polyploid H. parvula var. parvula individuals producing double reduced pollen, and the low levels of admixture detected, suggesting substantial post-pollination reproductive isolation between these two taxa. Nevertheless, caution should be taken when making strong taxonomic inferences from phylogenies that include R. baurii var. confecta and H. parvula var. parvula representatives from the Sentinel Peak study site.