Variation in chromosome number and breeding systems: implications for diversification in Pachycereus pringlei (Cactaceae)

Abstract

Polyploidy, the possession of more than two sets of chromosomes, is a major biological process affecting plant evolution and diversification. In the Cactaceae, genome doubling has also been associated with reproductive isolation, changes in breeding systems, colonization ability, and speciation. Pachycereus pringlei (S. Watson, 1885) Britton & Rose, 1909, is a columnar cactus that has long drawn the attention of ecologists, geneticists, and systematists due to its wide distribution range and remarkable assortment of breeding systems in the Mexican Sonoran Desert and the Baja California Peninsula (BCP). However, several important evolutionary questions, such as the distribution of chromosome numbers and whether the diploid condition is dominant over a potential polyploid condition driving the evolution and diversity in floral morphology and breeding systems in this cactus, are still unclear. In this study, we determined chromosome numbers in 11 localities encompassing virtually the entire geographic range of distribution of P. pringlei. Our data revealed the first diploid (2n = 22) count in this species restricted to the hermaphroditic populations of Catalana (ICA) and Cerralvo (ICE) Islands, whereas the tetraploid (2n = 44) condition is consistently distributed throughout the BCP and mainland Sonora populations distinguished by a non-hermaphroditic breeding system. These results validate a wider distribution of polyploid relative to diploid individuals and a shift in breeding systems coupled with polyploidisation. Considering that the diploid base number and hermaphroditism are the proposed ancestral conditions in Cactaceae, we suggest that ICE and ICA populations represent the relicts of a southern diploid ancestor from which both polyploidy and unisexuality evolved in mainland BCP, facilitating the northward expansion of this species. This cytogeographic distribution in conjunction with differences in floral attributes suggests the distinction of the diploid populations as a new taxonomic entity. We suggest that chromosome doubling in conjunction with allopatric distribution, differences in neutral genetic variation, floral traits, and breeding systems has driven the reproductive isolation, evolution, and diversification of this columnar cactus.

Floral morphology and reproductive biology in selected maple (Acer L.) species (Sapindaceae)

“…Furthermore, Acer species are mainly dichogamous, but other variants, including rare sexual systems (andromonoecy, androdioecy, andropolygamy, and trioecy) demonstrate the complex reproductive versatility of the genus. An intricate admixture of breeding systems, which along with variation in chromosome numbers and genetic diversity, has been reported as the drivers in the evolution and diversification of the columnar cactus Pachycerus pringlei (S. Watson) Britton & Rose (Gutiérrez-Flores et al 2018).…”

Induction of Polyploidy Using Colchicine in Flower Buds from Phalaenopsis Hybrids

“…In addition, plants of the same species usually have the same chromosomes number, but the chromosomes size and shape can different and varied. Variations are generally accompanied by numerical changes with insignificant differences in chromosomal size and shape [16]. These variations can have neutral, beneficial or damaging properties for plants [7].…”

Dioecy in Cactaceae: Species with Unisexual Flowers

“…Some of these are hybrids of different taxa, while others are considered "autopolyploids" which do not involve hybridization but instead some type of chromosome duplication. Gutiérrez-Flores et al (2018) noted various studies suggesting that polyploidy may lead some species to colonize new habitats that may have been hostile to diploid populations. They stated "For example, polyploidy in cacti allows the adaptation to freezing temperatures ...taxonomic diversification ...the colonization of higher latitudes ...wider geographical range ...and acts as a predictor of responses to environment and evolution."…”

“…Pachycereus pringlei has a remarkably large distribution; it is found throughout Baja California and along mainland Mexico. Gutiérrez-Flores et al (2018) examined eleven different populations from the entire range of the species. Two populations on Catalina Island and Cerralvo Island have perfect flowers.…”

“…Rimski (1954) reported that Cochemiea dioica is tetraploid. Gutiérrez-Flores et al (2018) found that all of the populations of Pachycereus pringlei with unisexual flowers were tetraploid, while the two populations with perfect flowers were diploid.…”