A mating system conundrum: hybridization in Apocynum (Apocynaceae)

Abstract

Based upon an intermediate morphology, Apocynum x-floribundum Greene has long been considered a hybrid involving A. androsaemifolium and A. cannabinum. The floral morphology in this genus, however, appears to prohibit both import and export of pollen, and observations of numerous insect visitors reveal that pollen is not routinely carried from flower to flower. Furthermore, reproductive success as measured by fruit set is very low in most populations. Hybridization was thus called into question, with allozyme evidence used to test the hypothesis of a hybrid origin for A. x-floribundum. Six diagnostic loci, as well as two loci exhibiting highly disparate allele frequencies, were resolved for each parent. All examined populations of A. x-floribundum were heterozygous at these loci, thus supporting the hypothesis of hybridization. Evidence from additional loci indicated that all populations tend to be strongly clonal. Observed heterozygosity was very low in the parental species, suggesting a history of inbreeding or a severe bottleneck. There was no support for earlier assumptions that some intermediates are derived from backcrosses or "secondary hybrids." Statistical analyses of plant height, leaf shape, petal length, sepal length, follicle length, seed length, and seed number per follicle supported these conclusions. The persistence and vegetative spread of hybrid clones may contribute to the illusion that hybridization is common.

The Biology of Canadian Weeds. 143. Apocynum cannabinum L.

“…Ransom et al (1998b) found that the genetic distance-based grouping of collections of this species were not highly correlated with the geographic location from which the plants originated, or with differences in morphological characteristics. In Colorado, however, 11 populations of A. cannabinum exhibited low heterozygosity at 11 loci with few polymorphisms at these loci; none contained more than two alleles (Johnson et al 1998). This low heterozygosity is unusual for an obligate outcrossing species, but may be due to the limited range (i.e., one state) from which the populations were sampled, relative to the much wider North America range of this species.…”

“…This low heterozygosity is unusual for an obligate outcrossing species, but may be due to the limited range (i.e., one state) from which the populations were sampled, relative to the much wider North America range of this species. Johnson et al (1998) speculated that this low heterozygosity indicated a long history of inbreeding or bottlenecking, but without random genetic drift. Ransom et al (1998b) found genetic variation among 16 populations from Michigan and Illinois was less than would be expected based on morphological differentiation.…”

“…Johnson et al (1998) stated that A. cannabinum usually grows in sandy or loamy soils; however, according to Robison and Jeffery (1972), the plant is seldom found in sandy soils with low fertility, and grows best in fertile, medium-or heavytextured soils. Dense infestations tend to be found in low-lying, wet areas, but fairly high densities may still occur on drier upland soils (Becker 1981;Voss 1996).…”

“…Apocynum cannabinum grows best in open spaces (Gleason and Cronquist 1991) and poorly in full shade (Balbach 1965). In non-agricultural settings, Muenscher (1951) found plants in gravelly fields, meadows, waste places, and along streams, whereas Johnson et al (1998) found plants mainly in prairie river flood plains, terraces, and roadside ditches.…”

“…The flower bud stage occurs approximately 4Á7 wk after shoot emergence (Schultz and Burnside 1979a;Becker 1981). In Colorado, Johnson et al (1998) observed that flowering began in mid-June (sometimes in May) and continued through August or until September. Plants do not flower in the first year after seedling emergence, but flower readily in the second season (Frazier 1945).…”