Black-fruited hawthorns of western North America — one or more agamic complexes?This paper is one of a selection of papers published in the Special Issue on Systematics Research.This paper is dedicated to the memory of the University of Idaho botanist who got us thinking about the complexities of Northwest hawthorns: the late Steven J. Brunsfeld.

Abstract

Black-fruited hawthorns in North America comprise two taxonomic groups within the genus Crataegus , section Brevispinae and section Douglasianae. The first of these has recently been shown to be monospecific, consisting of the blueberry haw, Crataegus brachyacantha Sarg. & Engelm., of Louisiana and Texas. Crataegus section Douglasianae, however, comprises several taxa in a single clade that is not closely related to section Brevispinae, and that is now one of the best-studied groups of hawthorns at least in North America. Most taxa in the group are found in, or west of, the Rocky Mountains. They include diploids, triploids, and tetraploids that can be ascribed to four or more species that differ in distribution and ecology, thorn morphology, leaf shape, and floral architecture. Diploids are self-incompatible, whereas polyploidy is associated with pseudogamous, gametophytic apomixis and self compatibility. Molecular data suggest that polyploids have arisen repeatedly, both within and from crosses between ploidy levels. We suggest that Crataegus section Douglasianae represents at least two agamic complexes that may serve as models for understanding the biology of, the relationships within, and the appropriate taxonomic treatment of other such groups that may make up much of the rest of the genus.

The classification of asexual organisms: Old myths, new facts, and a novel pluralistic approach

“…Usually they show a basic structure similar to the classical scheme of Babcock & Stebbins (1938) based on the North American Crepis agamic complex: a few sexual progenitor species hybridized and formed a huge mass of allopolyploid, apomictic derivatives. Several authors have proposed to separate out the sexual progenitor species that are usually found at the diploid or low polyploid level (Grant, 1981;Dickinson & al., 2008;Hörandl & al., 2009;Burgess & al., 2015). These diploid taxa are usually self-sterile, outcrossing and their primary hybrids often exhibit a low female and male fertility.…”

“…Recent RAD Seq studies on the complex, including more samples of R. ×hungaricus, rendered this species as highly polyphyletic (Karbstein & al., 2018). In the polyploids of Crataegus, phenetic clusters showed a great and often overlapping morphological variability (Dickinson & al., 2008). If such clusters were defined by morphology only, they may reflect an artificial assemblage of similar morphotypes of different evolutionary origins.…”

How apomictic taxa are treated in current taxonomy: A review

“…The Old World species are usually morphologically well-defined diploids, but common polyploidy and a high degree of hybridization blur clear species boundaries and the complex structure of populations still awaits elucidation. The situation with the New World species (especially the eastern North American species) is similar, but more complicated (Talent & Dickinson, 2005;Dickinson & al., 2008). Within the genus there are sexual self-incompatible diploids and facultatively asexual self-compatible taxa (mostly triploids and tetraploids) (Dickinson & al., 2008).…”

“…The situation with the New World species (especially the eastern North American species) is similar, but more complicated (Talent & Dickinson, 2005;Dickinson & al., 2008). Within the genus there are sexual self-incompatible diploids and facultatively asexual self-compatible taxa (mostly triploids and tetraploids) (Dickinson & al., 2008). Apomixis within the genus is apospory with pseudogamous endosperm development (Campbell & al., 1991).…”

“…Morphologically well-differentiated sexual diploids and/or tetraploids (which form a species series) are recognized as distinct species. Polyploid facultative apomictic taxa, of supposed hybrid origin, are assigned into a second group and are treated at the infraspecific level as microspecies or taxa within agamic complexes or a species series (Dickinson & al., , 2008. Such agamic complexes comprise a set of morphologically similar apomictic taxa.…”