Cytogeography and genome size variation in the Claytonia perfoliata (Portulacaceae) polyploid complex

Abstract

Background and aims: Genome duplication is a central process in plant evolution and contributes to patterns of variation in genome size within and among lineages. Studies that combine cytogeography with genome size measurements contribute to our basic knowledge of cytotype distributions and their associations with variation in genome size.

Methods: Ploidy and genome size were assessed with direct chromosome counts and flow cytometry for 78 populations within the Claytonia perfoliata complex, comprised of three diploid taxa with numerous polyploids that range to the decaploid level. The relationship between genome size and temperature and precipitation was investigated within and across cytotypes to test for associations between environmental factors and nuclear DNA content.

Key results: A euploid series (n = 6) of diploids to octoploids was documented through chromosome counts, and decaploids were suggested by flow cytometry. Increased variation in genome size among populations was found at higher ploidy levels, potentially associated with differential contributions of diploid parental genomes, variation in rates of genomic loss or gain, or undetected hybridization. Several accessions were detected with atypical genome sizes, including a diploid population of C. parviflora ssp. grandiflora with an 18 % smaller genome than typical, and hexaploids of C. perfoliata and C. parviflora with genomes 30 % larger than typical. There was a slight but significant association of larger genome sizes with colder winter temperature across the C. perfoliata complex as a whole, and a strong association between lower winter temperatures and large genome size for tetraploid C. parviflora.

Conclusions: The C. perfoliata complex is characterized by polyploids ranging from tetraploid to decaploid, with large magnitude variation in genome size at higher ploidy levels, associated in part with environmental variation in temperature.

Fig. 1.

Map of the western USA depicting locations of diploid, tetraploid, hexaploid and octoploid cytotypes of the Claytonia perfoliata complex identified in this study. Precise locations of all populations are provided in the Supplementary Data Table S1.

Fig. 2.

Representative chromosome counts (mitotic) and flow cytometry results for the C. perfoliata complex. (A) Hexaploid C. perfoliata ssp. perfoliata. (B) Diploid C. rubra ssp. rubra. (C) Diploid C. parviflora ssp. grandiflora, 2n = 12. (D) Tetraploid C. parviflora ssp. parviflora, 2n = 24. (E) Octoploid C. perfoliata ssp. perfoliata, 2n = 48. Scale bars = 10 µm.

Fig. 3.

Genome size (2C) estimates from flow cytometry for all individuals in this study, for C. perfoliata, C. parviflora and C. rubra. Symbols depict ploidy status as determined by direct chromosome counts, or whether values are from individuals without direct chromosome counts. Note the overlap in genome size between hexaploids and octoploids among individuals, preventing clear determination of ploidy from flow cytometry at these ploidy levels.