Nadia Talent

The potential for ploidy level increases and decreases in Crataegus (Rosaceae, Spiraeoideae, tribe Pyreae)

Unlike their diploid relatives, some triploid and tetraploid Crataegus frequently produce unreduced megagametophytes. In all cases, pollination is required for successful seed set, but in polyploids, endosperm formation can involve fertilization by either one or both sperm. Apomixis, in which the egg develops parthenogenetically, is widely documented in polyploid Crataegus, and as in many other groups with gametophytic apomeiosis, fertilization of unreduced eggs can also occur. Reciprocal pollinations were made between diploids, triploids, and tetraploids belonging to five taxonomic series in the genus to evaluate opportunities for gene flow between ploidy levels. The ploidy levels of embryo and endosperm in mature seeds, estimated from flow-cytometric DNA measurements, indicate the meiotic or apomeiotic origin of the megagametophyte and whether fertilization has occurred. These experiments demonstrated that although some tetraploids maintain near-obligate apomixis when supplied with pollen from diploids,...

Evolution of gametophytic apomixis in flowering plants: an alternative model from Maloid Rosaceae

Gametophytic apomixis, asexual reproduction involving megagametophytes, occurs in many flowering-plant families and as several variant mechanisms. Developmental destabilization of sexual reproduction as a result of hybridization and/or polyploidy appears to be a general trigger for its evolution, but the evidence is complicated by ploidy-level changes and hybridization occurring with facultative apomixis. The repeated origins of polyploid apomictic complexes in the palaeopolyploid Maloid Rosaceae suggest a new model of evolutionary transitions that may have wider applicability. Two conjectures are fundamental to this model: (1) that as previously suggested by Rutishauser, like many sexual flowering plants the polyploid apomicts require maternal–paternal balance in the second fertilization event that gives rise to the endosperm, and (2) that the observed variation in endosperm ploidy levels relates less to flexibility late in development than to the known variation in developmental origin of the megagametophyte between mechanisms loosely categorized as diplospory and apospory. The model suggests explanations for the relative frequencies of apospory and diplospory, and for the wide but incomplete associations of apospory with a pollination requirement (pseudogamy) and of diplospory with autonomous development of the endosperm. It is suggested that pollination from other taxa may provide some adaptive advantage to pseudogamous apospory.

Effects of apomixis and polyploidy on diversification and geographic distribution in Amelanchier (Rosaceae)

UNLABELLED • PREMISE OF THE STUDY Amelanchier polyploid apomicts differ from sexual diploids in their more complex diversification, greater species problems, and geographic distribution. To understand these differences, we investigated the occurrence of polyploidy and frequency of apomixis. This research helps clarify species delimitation in an evolutionarily complex genus.• METHODS We used flow cytometry to estimate genome size of 1355 plants. We estimated the frequency of apomixis from flow-cytometrically determined ploidy levels of embryo and endosperm and from a progeny study using RAPD markers. We explored relationships of triploids to other ploidy levels and of ploidy levels to latitude plus elevation.• KEY RESULTS Diploids (32% of sample) and tetraploids (62%) were widespread. Triploids (6%) mostly occurred in small numbers with diploids from two or more species or with diploids and tetraploids. Seeds from diploids were 2% apomictic, the first report of apomixis in Amelanchier diploids. Seeds from triploids were 75% apomictic. We documented potential triploid bridge and triploid block from unbalanced endosperm and low pollen viability. Seeds from tetraploids were 97% apomictic, and tetraploids often formed microspecies. We did not find strong evidence for geographical parthenogenesis in North American Amelanchier. Most currently recognized species contained multiple ploidy levels that were morphologically semicryptic.• CONCLUSIONS Documentation of numerous transitions from diploidy to polyploidy helps clarify diversification, geographic distribution, and the species problem in Amelanchier. Despite the infrequent occurrence of triploids, their retention of 25% sexuality and capacity for triploid bridge may be important steps between sexual diploids and predominantly apomictic tetraploids.

DNA barcodes from four loci provide poor resolution of taxonomic groups in the genus Crataegus

The leaves and fruits of some of the approximately 230 species of hawthorn (Crataegus) yield natural health products with significant therapeutic effects on symptoms of cardiovascular disease. DNA barcoding could be a valuable tool for authenticating these products, but only a small fraction of the 93 taxa that we examined were distinguished, even though all major clades and eight out of ten taxonomic sections of the genus were included. DNA barcoding as currently practised thus has limited utility in Crataegus. Hybridization, lineage sorting due to incomplete concerted evolution in ITS2, and limited variation in plastid loci are implicated.

Crataegus ×ninae-celottiae and C. ×cogswellii (Rosaceae, Maleae), two spontaneously formed intersectional nothospecies

Abstract Crataegus monogyna Jacq. is naturalized in North America, where it has hybridized with native diploid hawthorns at least twice. We provide names for the two nothospecies (as well as for the corresponding nothosections and nothoseries), referring to existing documentation in the literature for nothosp. nov. Crataegus ×ninae-celottiae K.I. Chr. & T.A. Dickinson (C. monogyna × C. punctata Jacq.). New data are provided to further document nothosp. nov. Crataegus ×cogswellii K.I. Chr. & T.A. Dickinson (C. monogyna × C. suksdorfii (Sarg.) Kruschke). In both cases, the striking differences in leaf shape between most New World hawthorns and Old World section Crataegus, and the intermediacy of the hybrids, account for the relative ease with which these hybrids can be recognized. Finally, new sequence data from ITS2 and chloroplast DNA barcoding loci confirm the genetic relationships between the two nothospecies and their respective parents.