How Opuntia Species Are Identified

Introduction

Opuntia polyacantha, NM
Opuntia polyacantha, New Mexico

The first iteration of Opuntia Web was built in 2005. Our scientists identified species in the field. We visited the western states to study cacti in habitat. We also visited Florida, the southern Atlantic states, and the Midwest to observe cacti in those regions (e.g., Opuntia macrarthra). We made a visit to Pennsylvania to verify the presence of Opuntia. In addition to extensive field studies, we consulted herbarium specimens, old black and white photographs, compendia, the current literature, and original citations and descriptions.

Bentonii with Fruit
Opuntia bentonii with Fruit, Galveston, TX

We compared our field observations with the information we found from these sources to identify the plants that we observed. Additionally, we consulted with other specialists and scientists an ad hoc basis. Herein, we provide evidence to support our conclusions in the form of descriptions, published literature, photo-reproductions of herbarium samples, and in situ photographs. Opuntia Web does not provide new taxonomic decisions. Rather, it uses published descriptions of species and identifies the plants in the field. Taxonomy is a separate discipline from identification of plants with published materials. 

The understanding of Opuntia continues to evolve, and there are many names available for some species. A single citation is used herein to provide a taxon name. The precise citation chosen provides the earliest identifying name for the taxon as we treat it and a place for the reader to start if more information is desired.

We anticipate that some names will change in the future. We also understand that currently undescribed taxa will be reported in the future. We will change our treatments accordingly as new information is published. For instance, one of our editors (Nancy Hussey) recently co-published the description of O. diploursina with her colleagues.

Some species of Opuntia have been lost to science because they were assumed to be synonymous with other taxa. For instance, O. cespitosa was lost because it was assumed to be synonymous with O. humifusa. However, more recently, botanists have shown O. cespitosa to be separate and distinct from O. humifusa. Likewise O. mesacantha ssp. mesacantha and O. mesacantha ssp. lata were assumed to be synonymous with O. humifusa but are now known to be different. Thus, there are four species known were it was thought there was only one. 

What is a species?

Opuntia valida, AZ
Opuntia aff valida, AZ

Identification of species requires a working sense of what a species is. We use the following considerations as our guide, “clusters of monotypic or polytypic biological entities, identified using morphology, genetics, or molecular biology, forming groups that have few or no intermediates with other groups when in contact.” We also accept that species are “groups of organisms that inherit characters from each other…and are a reproductive community and unit.” 

We know that future phylogenetic hypotheses supported by molecular comparisons and rigorous morphometric and ecological analyses (see the treatment of O. humifusa s.l.) will supplement our current observations and help further illuminate our  knowledge of the genus. Field studies will be essential for our understanding.

We consider varieties as populations within a species that differ from the type in some details and that blend into one another anywhere their distributions meet or overlap. It is often difficult to label a plant to a proper variety because, unlike hybrids, varietal intermediates may occur with regularity and may be only subtly different from the type. Our thought is that widespread and variable species (e.g., O. phaeacantha or O. engelmannii) may contain undescribed varieties or even subspecies and species. For example O. gregoriana is now considered to be a form of O. engelmannii, but perhaps it could be described as a variety. 

Opuntia wootonii flower
Opuntia wootonii flower, Belen, NM

Details

We consider gross morphological information such as spine colors, sizes, numbers, and arrangements, etc; glochid arrangements, sizes, and colors; cladode shape, color, and overall aspect; areole details; fruit and seed characteristics, general plant appearance, size, and shape; etc. Even edaphics, elevation, geography, and climate are considered as we identify species. Sometimes bloom season is considered. If observable, we also consider phenotypic clines, introgressions, and other types of data in making decisions about the limits of a taxon. Ploidies are determined by our Editor-in-Chief or are reported in the literature

Other data, such as phenology; pollination ecology; ecosystem position, and proposed phylogenies; etc., are mostly not available for opuntias at the specific level for the taxa we recognize. However, where published such information is used to inform our treatments and descriptions.

Because opuntias are notoriously plastic in gross morphology, we do not rely upon single plants, observations, or locations, etc., to inform our decisions. Rather, we observe multiple plants, in multiple locations, under different conditions, and over different seasons to make determinations.

Biology

Opuntia is a dynamic genus, and  hybridization, polyploidy, and introgression are important biological processes that affect opuntias as well as our understanding of them. 

Hybrids

Opuntia hybrids do occur, but they are not the norm even though they can be confusing. Given the abundance of Opuntia species, the sheer number of individual plants, and their overlapping ranges, there are actually few hybrids observed in Nature. When they do occur they are generally found in hybrid zones or regions where two taxa overlap. Even in such zones, Opuntia hybrids are the exception. Because they are exceptions, and may have unique features, hybrids are often prized in gardens. We generally do not describe a taxon unless it can be reliably and consistently differentiated in the field. Therefore, we do not describe most Opuntia hybrids. 

Polyploidy

Polypoidization occurs in Opuntia. The basic chromosome number for Opuntia is 11 (2N =22) However, many Opuntia species are polyploid; they may be tetraploid (2N = 44), hexaploid (2N = 66), or even octaploid (2N = 88). Triploid (N = 3X) and pentaploid (N = 5X) plants are known, but their contributions to further gene exchange are unknown. 

Opuntia phaeacantha
Opuntia phaeacantha

Polyploidy may mask entities that are apparently identical, but which actually have different ploidies. For instance, there are diploid, triploid, and tetraploid examples of O. pusilla. Similarly, there are diploid and tetraploid examples of O. humifusa s.l. Auto- and allopolypoidy occurs in Opuntia, and such rich genetic resources have created wonderfully varied species. 

Introgression

Introgression occurs in Opuntia species. Introgression is different from hybridization because it refers to the transfer of limited genetic information from one species to another through ancient hybridization and repeated backcrossing. Introgression begins with hybridization, but it is so much more. Hybridization may split two genomes equally, whereas introgression is generally the invasion of a few genes into one of the parents–leaving the species essentially the same as before. 

Introgression is a ubiquitous phenomenon in plants and animals and it contributes to adaptation. Modern humans are an example of an introgressed species because many of us carry some genes from Neanderthal humans. An example of introgression in Opuntia might be a pink-flowered population in a species that is normally yellow flowered.

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