Spontaneous hybrids between native and exotic Rubus in the Western United States produce offspring both by apomixis and by sexual recombination

Abstract

Facultative asexual reproduction is a trait commonly found in invasive species. With a combination of sexual and asexual reproductive modes, such species may adapt to new environments via sexual recombination during range expansion, while at the same time having the benefits of asexuality such as the maintenance of fitness effects that depend upon heterozygosity. In the Western United States, native species of Rubus (Rosaceae) reproduce sexually whereas exotic naturalized Rubus species reproduce by pseudogamous apomixis. We hypothesized that new asexual lineages of Rubus could arise from hybridization in this range. To detect hybridization between native and exotic Rubus, we genotyped 579 individuals collected across California, Oregon and Washington with eight nuclear microsatellites and two chloroplast markers. Principal Coordinate Analysis and Bayesian clustering revealed a limited amount of hybridization of the native R. ursinus with the exotic R. armeniacus and R. pensilvanicus, as well as cultivated varieties. Genetic distances between these hybrids and their offspring indicated that both R. ursinus  R. armeniacus and R. ursinus  R. pensilvanicus produced a mix of apomictic and sexual seeds, with sexual seeds being more viable. Although neither of these hybrid types is currently considered invasive, they model the early stages of evolution of new invasive lineages, given the potential for fixed heterosis and the generation of novel genotypes. The hybrids also retain the ability to increase their fitness via sexual recombination and natural selection. Mixed reproductive systems such as those described here may be an important step in the evolution of asexual invasive species.

Melting pot of biodiversity: first insights into the evolutionary patterns of the Colchic bramble flora (RubussubgenusRubus, Rosaceae)

“…They can also confirm recent hybridization events (e.g. Clark & Jasieniuk, ; M. Sochor and B. Trávníček, unpubl. data).…”

Comparison of the Micromorphology and Ultrastructure of Pollen Grains of Selected Rubus idaeus L. Cultivars Grown in Commercial Plantation

“…The genus Rubus is taxonomically complex and exhibits high morphological diversity. The characterization and infrageneric classification of its species are complicated by frequent processes of hybridization, polyploidy, and agamospermy [53][54][55][56][57][58]. Although some species from the Rosaceae family are stenopalynous plants, e.g., Rubus and Potentilla, [59][60][61][62], they vary in the structure of pollen grains, which are used as indicators of taxonomic affiliation [60,[63][64][65][66].…”

The Biology of Canadian Weeds. 154. Himalayan blackberry

“…This suggests an asexual mode of seed production with rapid vegetative dispersal of a single genotype. However, hybrids between Himalayan blackberry (as R. armeniacus) and R. ursinus do have the potential to produce genetically diverse offspring (Clark and Jasieniuk 2012).…”

“…The R. ursinus)R. armeniacus hybrids could be distinguished by their leaf, stem and prickle morphology that is intermediate to that of the parental species. Hybrids produced both apomictic and sexual seeds, with the latter being more viable (Clark and Jasieniuk 2012). They hypothesized that the mixed mating system of the hybrids might facilitate increased invasiveness, with the retention of the ability to produce recombinant offspring.…”