Disparate Style Lengths Contribute to Isolation of Species in Rhododendron

Abstract

For a group of closely related Rhododendron species in section Vireya, it is shown that success of interspecific pollinations is significantly dependent on male/female style length ratio (SLR), and apparently independent of whether both species belong to the same taxonomic subsection. Crosses with SLR < 0.2 or SLR > 6 were unsuccessful, with the probability of success increasing as SLR approached 1.0. When SLR was < 0.2 pollen tubes of the shorter-styled species were unable to reach the ovary. When SLR was > 5-6, pollen tubes of the longer-styled species penetrated deeply into the ovary but were rarely observed to enter ovules. Disparate style lengths are therefore a major incongruity factor among Vireya rhododendrons, and may contribute to species isolation.

Unilateral Cross-Incompatibility in Eucalyptus: the Case of Hybridisation Between E. globulus and E. nitens

“…In Rhododendron, the success of interspecific pollinations was dependant on the male/female style length ratio, and where the style length ratio was <0.2, pollen tubes of the shorter styled species were unable to reach the ovary. Williams and Rouse (1988) consider that in such cases pollen tube growth might be due to limited nutritional reserves within the pollen grain, combined with limited access to stylar reserves or alternatively, pre-programming of the pollen grain for finite growth correlated with pistil size, hypotheses which are difficult to differentiate in the present case.…”

“…Nevertheless, to date attempts to break the unilateral barrier to hybridisation between E. globulus and E. nitens by artificially shortening the E. globulus styles have not been successful with neither the E. globulus control nor the E. nitens pollinations setting seed (Badcock and Volker, unpublished data). Reduced cross success was also reported in Rhododendron when some large-flowered species were used to pollinated small-flowered species, possibly due to mis-matched timing of male and female maturity (Williams and Rouse 1988) or overgrowth of pollen tubes (Williams et al 1986). Such an effect has not as yet been reported in Eucalyptus where small-flowered species have been successfully pollinated by quite large-flowered species (e.g.…”

“…As in the present case, this barrier is frequently asymmetrical resulting in the unilateral failure of interspecific hybridisation when small flowered species are used as pollen parents e.g. Prunus (Perez and Moore 1985) and Rhododendron (Williams and Rouse 1988). In Rhododendron, the success of interspecific pollinations was dependant on the male/female style length ratio, and where the style length ratio was <0.2, pollen tubes of the shorter styled species were unable to reach the ovary.…”

“…The inability of the pollen tubes of the small-styled species to grow the full length of the larger style has been suggested as the simplest explanation for the unilateral failure of hybridisation (Pryor 1956;Tibbits 1986;Potts and Savva 1988). This certainly appears to be the case for crosses involving species with comparable disparities in style length in other plant genera such as Datura (Gardella 1950), Phlox (Levin 1978a) and Rhododendron (Williams and Rouse 1988). Nevertheless, unilateral failure of hybridisation may arise through S-locus incompatibility where self-incompatible (SI) species are crossed with self-compatible (SC) species (Lewis and Crowe 1958;de Nettancourt 1977) or incongruity (Hogenboom 1975(Hogenboom , 1984 operative at either a pre-zygotic (e.g.…”

“…Post-mating barriers to interspecific hybridisation within plant genera are often unilateral (Lewis and Crowe 1958;Abdalla and Hermsem 1972;Hogenboom 1975Hogenboom , 1984de Nettancourt 1977de Nettancourt , 1984Levin 1978a;Williams and Rouse 1988) and Eucalyptus is no exception. While hybridisation between species within Eucalyptus subgenera is relatively common (Griffin et al 1988), the unilateral failure of artificial hybridisation within subgenera has been reported for E. caesia x E.pulverulenta (Pryor 1956), E. caesia x E. leucoxylon (Beardsell et al 1979), E. globulus x E. viminalis (Pryor 1956), E. globulus x E. nitens (Tibbits 1986), E. globulus x E. ovata, E. globulus x E. gunnii and E. globulus x E. morrisbyi (Potts and Savva 1988).…”