Cytogeography of the Humifusa clade of Opuntia s.s. Mill. 1754 (Cactaceae, Opuntioideae, Opuntieae): correlations with pleistocene refugia and morphological traits in a polyploid complex

Abstract

Ploidy has been well studied and used extensively in the genus Opuntia to determine species boundaries, detect evidence of hybridization, and infer evolutionary patterns. We carried out chromosome counts for all members of the Humifusa clade to ascertain whether geographic patterns are associated with differences in ploidy. We then related chromosomal data to observed morphological variability, polyploid formation, and consequently the evolutionary history of the clade. We counted chromosomes of 277 individuals from throughout the ranges of taxa included within the Humifusa clade, with emphasis placed on the widely distributed species, Opuntia humifusa (Raf.) Raf., 1820 s.l. and Opuntia macrorhiza Engelm., 1850 s.l. We also compiled previous counts made for species in the clade along with our new counts to plot geographic distributions of the polyploid and diploid taxa. A phylogeny using nuclear ribosomal ITS sequence data was reconstructed to determine whether ploidal variation is consistent with cladogenesis. We discovered that diploids of the Humifusa clade are restricted to the southeastern United States (U.S.), eastern Texas, and southeastern New Mexico. Polyploid members of the clade, however, are much more widely distributed, occurring as far north as the upper midwestern U.S. (e.g., Michigan, Minnesota, Wisconsin). Morphological differentiation, although sometimes cryptic, is commonly observed among diploid and polyploid cytotypes, and such morphological distinctions may be useful in diagnosing possible cryptic species. Certain polyploid populations of Opuntia humifusa s.l. and Opuntia macrorhiza s.l., however, exhibit introgressive morphological characters, complicating species delineations. Phylogenetically, the Humifusa clade forms two subclades that are distributed, respectively, in the southeastern U.S. (including all southeastern U.S. diploids, polyploid Opuntia abjecta Small, 1923, and polyploid Opuntia pusilla (Haw.) Haw., 1812) and the southwestern U.S. (including all southwestern U.S. diploids and polyploids). In addition, tetraploid Opuntia humifusa s.l., which occurs primarily in the eastern U.S., is resolved in the southwestern diploid clade instead of with the southeastern diploid clade that includes diploid Opuntia humifusa s.l. Our results not only provide evidence for the polyphyletic nature of Opuntia humifusa and Opuntia macrorhiza, suggesting that each of these represents more than one species, but also demonstrate the high frequency of polyploidy in the Humifusa clade and the major role that genome duplication has played in the diversification of this lineage of Opuntia s.s. Our data also suggest that the southeastern and southwestern U.S. may represent glacial refugia for diploid members of this clade and that the clade as a whole should be considered a mature polyploid species complex. Widespread polyploids are likely derivatives of secondary contact among southeastern and southwestern diploid taxa as a result of the expansion and contraction of suitable habitat during the Pleistoce...

Polymorphism analysis in some Algerian <em>Opuntia</em> species using morphological and phenological UPOV descriptors

“…The use of morphological descriptors comprising cladode, flower and fruit traits yielded a high number of morphotypes and allowed the discrimination of all of the studied species. In previous studies, the discrimination level was found to be as high as those reported by other studies using molecular markers for Opuntia spp (Labra et al 2003, García-Zambrano et al 2006, Zoghlami et al 2007, García-Zambrano et al 2009, Caruso et al 2010, Majure et al 2012, El-Banna et al 2013, Valadez-Moctezuma et al 2014, Ganopoulos et al 2015, Samah et al 2016.…”

“…Generally, the characterization of Opuntia genus is incomplete due to few morphological characters taken into account to classify them into the genus (Rebman & Pinkava 2001); also, the inadequate descriptors have led to misclassification (Caruso et al 2010, Majure et al 2012, they usually lack botanical references and other data, which makes it difficult to properly document and register different varieties under the basis of a reliable classification procedure (Gallegos-Vázquez 2012). Several authors have analyzed morphological diversity in several plants -Vázquez 2012).…”

Genetic divergence between Mexican Opuntia accessions inferred by polymerase chain reaction-restriction fragment length polymorphism analysis

“…Moreover, species limits are still poorly understood because of the high frequency of polyploid taxa. Therefore, it seems that chromosomal data are of little value for infrageneric classification (Majure et al, 2012a). Opuntia has been recorded as diploid to enneaploid (2n = 9x = 99), with 60.4% of reported counts pertaining to polyploidy individuals and another 13.4% representing taxa with both diploid and polyploid cytotypes (Majure et al, 2012a).…”

“…Furthermore, there is no comprehensive phylogeny of Opuntia s.s., so the limits of major clades are largely unknown. Numerous morphological and cytological studies have been conducted on large groups of taxa and species complexes (e.g., Majure et al, 2012a). Griffith and Porter (2009) included 28 species of Opuntia s.s. in their molecular phylogeny of the Opuntioideae, but were unable to resolve relationships within Opuntia s.s. using nuclear internal transcribed spacers (ITSs) and the plastid intergenic spacer trnL-F. Hernández-Hernández et al (2011) and Bárcenas et al (2011) studied phylogenetic relationships between South American Opuntia species, but only surveyed seven species and had no resolution between clades, respectively.…”

Phylogeny of Korean Opuntia spp. based on multiple DNA regions

“…Although there have been several studies on the phylogeny of Opuntia (Griffith and Porter., 2009;Hernandez et al, 2011;Majure et al, 2012aMajure et al, , 2012b, Korean Opuntia spp. have not been included in these studies, and so their taxonomic position is not known.…”