Influence of polyploidy on morphology, genetic differentiation and reproductive strategy amongst varieties of Rhodohypoxis baurii (Hypoxidaceae)

Abstract

Evolutionary mechanisms, such polyploidy (increase in chromosome sets), alters plant morphology, gene flow and reproductive strategies, which can facilitate the generation or loss of species. Rhodohypoxis L. (Hypoxidaceae) is a small near-endemic Drakensberg genus comprising six species, one of which is Rhodohypoxis baurii. Rhodohypoxis baurii contains three morphologically distinct varieties, with varying ploidy-levels: R. baurii var. baurii (2×, 4×), R. baurii var. platypetala (2× 3× , 4×), and R. baurii var. confecta (2×). Therefore, R. baurii is an ideal system to evaluate whether polyploidy leads to lineage divergence or homogenization and contributes to biodiversity in this lineage. The aim of the study was to assess the influence of ploidy on morphology, genetic differentiation and reproductive strategy among varieties of Rhodohypoxis baurii (Hypoxidaceae), as well as to better recognise the three varieties. Thirty vegetative, floral, and reproductive traits were measured across 124 herbarium specimens and 43 individuals housed in the greenhouse. A matrix containing 20 quantitative and 12 qualitative characters was constructed and a Principal Coordinates Analysis (PCoA) and Non-Metric Multidimensional Scaling analysis (NMDS) were conducted. Important distinguishing morphological features that had a high eigenvalue (shown via a PCA) were selected for direct comparison using box and whisker box plots to compare means ± standard errors (SE). Certain morphological traits such as anther length, peduncle length and tepal sizes differed significantly among the varieties and ploidy levels, with polyploid individuals exhibiting the gigas effect. This was especially evident in R. baurii var. platypetala, which contained many polyploid individuals and exhibited larger flowers (longer and wider tepals) and larger anthers compared to other varieties. Rhodohypoxis baurii var. confecta and R. baurii var. platypetala are genetically, geographically and morphologically similar, differing only in flower colour, flower size and peduncle length. However, most of these differences can be attributed to differences in ploidy and /or altitude with R. baurii var. confecta occurring at higher altitudes and R. baurii var. platypetala containing multiple ploidy-levels. It is therefore evident that ecological differences and polyploidy have shaped the morphological differences in these two taxa. In addition, R. baurii var. baurii populations in the 7 Eastern Cape Drakensberg were morphologically, geographically and genetically distinct from all other populations and varieties, and may be a new/ undescribed taxon; however, this warrants further investigation. Out of 231 experimental crosses, 113 intra-varietal and inter-varietal crosses produced seeds. Rhodohypoxis baurii polyploid individuals show a shift away from sexual reproduction to asexual reproduction as they all showed higher rates of clonal reproduction than the diploid individuals. Moreover, crosses between polyploids yielded lower seed sets and lower germination rates than diploid-diploid crosses. Genetic differentiation and gene flow were quantified for 280 individuals among the varieties and ploidy-levels across 11 populations using 12 microsatellite markers labelled with the FAM NED dyes. Leaf material was collected from 237 individuals of Rhodohypoxis baurii (R. baurii var. confecta n = 88, R. baurii var. baurii n = 87, R. baurii var. platypetala n = 62) and flow cytometry conducted to estimate ploidy. A latitudinal ploidy gradient was evident across sampled populations that corresponds with shifts in reproductive strategy, and changes in the extent of gene flow. Population genetic structure coincided primarily with geographic localities, with diploid Northern Drakensberg populations having similar allelic diversity to one another. The Central and Southern Drakensberg mixed ploidy populations also showed similar allelic diversity but differed from the tetraploid Eastern Cape Drakensberg populations. Furthermore, gene flow was higher between geographically close populations irrespective of ploidy-level, with geographically isolated regions (such as the Eastern Cape Drakensberg) and outlying populations (i.e. Karkloof) showing unique genotypes, indicating little gene flow and allele sharing. Consequently, shifts in reproductive strategy and geographic isolation are likely changing gene flow patterns among varieties and ploidy levels which appears to be facilitating both lineage diversification and homogenization in this species