The morphometrics of autopolyploidy: insignificant differentiation among sexual–apomictic cytotypes

Abstract

Polyploidization of the plant genome affects the phenotype of individuals including their morphology, i.e. size and form. In autopolyploids, we expect mainly nucleotypic effects, from a number of monoploid genomes (i.e. chromosome sets) or genome size, seen from an increase in size or dimension of the polyploids compared with the diploids (or lower ploids). To identify nucleotypic effects, confounding effects of hybridity (observed in allopolyploids), postpolyploidization processes or environmental effects need to be considered. We morphometrically analysed five ploidy cytotypes of the sexual–apomictic species Potentilla puberula cultivated ex situ under the same experimental conditions. Sexuals are mainly tetraploid, while higher ploidy (penta- to octoploidy) is typically associated with the expression of apomixis. The cytotypes likely arose via autopolyploidization although historic involvement of another species in the origin of apomicts cannot be fully ruled out, suggested by a slight molecular differentiation among reproductive modes. We (i) revisited molecular differentiation using amplified fragment length polymorphisms and performed a morphometric analysis to test (ii) if cytotypes are morphologically differentiated from each other and (iii) if the size of individuals is related to their ploidy. Weak molecular differentiation of sexual versus apomictic individuals was confirmed. Cytotypes and reproductive modes were also morphologically poorly differentiated from each other, i.e. apomicts largely resampled the variation of the sexuals and did not exhibit a unique morphology. Overall size of individuals increased moderately but significantly with ploidy (ca. 14 % in the comparison of octo- with tetraploids). The results support an autopolyploid origin of the P. puberula apomicts and suggest a nucleotypic effect on overall plant size. We discuss taxonomic consequences of the results in the context of data on reproductive relationships among cytotypes and their ecological preferences and evolutionary origin, and conclude that cytotypes are best treated as intraspecific variants within a single species.

Single Independent Autopolyploidization Events From Distinct Diploid Gene Pools and Residual Sexuality Support Range Expansion of Locally Adapted Tetraploid Genotypes in a South American Grass

“…The variation rather exhibits a continuum of morphological character distribution among all the populations that is rooted in the variation observed for three representatives of the diploid cytotype (Figure S3). While lack of morphologically discrete cytotype groups can be expected in grass species (e.g., Amirouche and Misset, 2007 ; Pimentel and Sahuquillo, 2008 ), or in other families (e.g., in the Rosaceae; Bigl et al, 2019 ), cases of significant morphological differentiation between cytotypes have been observed in many other plant groups (e.g., Segraves et al, 1999 ; Kao and Parker, 2010 ), including one species from the genus Paspalum ( Quarin and Hanna, 1980 ). Such morphological variation is mostly due to nucleotypic effects (i.e., effects associated with the DNA content in nuclei or the number of monoploid genomes) or to confound phenomena like hybridity (in allopolyploids) or ecological differentiation ( Ramsey and Schemske, 2002 ).…”