North American black-fruited hawthorns. I. Variation in floral construction, breeding system correlates, and their possible evolutionary significance inCrataegus sect.Douglasii London

Reconstructing Reticulation History in a Phylogenetic Framework and the Potential of Allopatric Speciation Driven by Polyploidy in an Agamic Complex in Crataegus (Rosaceae)

“…1–4). These 3 x individuals also display a relatively lower haplotype diversity than the others (Table 2), which is to be expected, considering that these triploids are either sterile or less fertile than diploids and tetraploids (Dickinson et al 1996). Both the nuclear and organelle genomes of the Oregon triploids were inherited only from the 2 x C. suksdorfii with no other taxa involved (Fig.…”

“…In Crataegus , the preponderance of evidence supports this scenario, because pollen from diploids typically is reduced (Talent and Dickinson 2007), even though cases of unreduced mate gametes in diploids are seen in other groups (Orjeda et al 1990; Maceira et al 1992; Ortiz et al 1992; Ramsey 2007). Moreover, unreduced female gametes produced by these 3 x individuals (Dickinson et al 1996), and gametophytic apomixis generally provide a mechanism for both a degree of reproductive isolation as well as means of reproductive assurance if pollen and/or pollinator services are available.…”

“…Triploids are widespread and locally abundant at higher elevations and further east in Washington and Idaho. Tetraploids, however, are of sporadic occurrence (Dickinson et al 1996; Lo et al 2009a). Over parts of their range, polyploid cytotypes of C. suksdorfii are sympatric with exclusively tetraploid C. douglasii .…”

Endosperm formation in aposporous Crataegus (Rosaceae, Spiraeoideae, tribe Pyreae): parallels to Ranunculaceae and Poaceae

“…Earlier experiments have shown that pollination is required for seed formation in apomictic tetraploid Crataegus (Dickinson & Phipps, 1986; Smith & Phipps, 1988; Dickinson et al ., 1996), including C. crus‐galli var. pyracanthifolia as used here.…”

Effects of pollen quality and quantity on pollen limitation in Crataegus monogyna (Rosaceae) in NW Spain

“…Flowers grow in small clusters of 9-18 (Tutin et al, 1968) and are entomophilous of the 'generalist' bowl form (Proctor et al, 1996). Crataegus species are unique in the Maloideae in having two ovules that are typically superposed within the locule (Dickinson et al, 1996;Sterling, 1964). In northern Spain, the plant flowering season is April-June.…”