Pollinator behaviour, mate choice and the realised mating systems of Grevillea mucronulata and Grevillea sphacelata

Abstract

Successful long-term conservation and management of populations of plants requires successful management of the suite of factors that determine their ‘realised’ mating systems. Within the genus Grevillea, mating systems are potentially complex. They may vary among species and among populations within a species, reflecting variation in pollinator behaviour and diversity (‘potential’ mating systems) and in breeding system (the ‘preferred’ mating system). We used a combination of pollinator observations, pollination experiments and electrophoretic analysis of seed from open pollinations, to examine variation in potential and preferred mating systems in two populations of each of two ‘spider-flowered’ Grevillea species: G. mucronulata (visited by honeyeaters and honeybees, although only birds effected pollen removal and transfer) and G. sphacelata (visited only by honeybees, which made frequent contact with pollen and stigmas). Almost all observed bird and insect foraging bouts on either species involved movements among inflorescences within plants or among inflorescences on closely neighbouring plants. On the basis of these data, the mating system was predicted to involve a high level of selfing or inbreeding. However, the pollination experiment revealed that both species were highly self-incompatible and showed a clear preference for outcross pollen in mate-choice experiments. For both species, fruit set through autogamy was lower than 0.7% and fruit set from self-pollinations was always significantly lower than for outcross pollinations (0–11% cf. 25–33% for G. mucronulata and 0% cf. 4.2–8.8% for G. sphacelata). Allozyme studies revealed that genotypes in open-pollinated seeds on 20 G. mucronulata and 20 G. sphacelata plants were surprisingly uniform, best explained by outcrossed matings between close neighbours. We found little between-population variation in any aspect of the mating system for either species. These studies reveal that the preferred mating system of the plant and the pattern of pollinator behaviour interact to determine the mating system in a population, emphasising the need for a multifaceted investigation of mating systems, especially in predicting the fates of populations that have pollination systems altered by disturbance, small size, isolation and introduced species such as the honeybee.

A Molecular Genetic Assessment of Mating‐System Variation in a Naturally Bird‐Pollinated Shrub: Contributions from Birds and Introduced Honeybees

“…Second, bees are often observed taking only nectar, leaving an undisturbed pollen cap on the pollen presenter (which surrounds the stigma). Richardson et al (2000) reported that honeyeaters contact pollen presenters of G. mucronulata on 98% of visits, whereas honeybees do so on only 18.5% of visits. Third, birds typically display more among‐plant than within‐plant movements.…”

The accumulation of genetic diversity within a canopy‐stored seed bank

“…Fruit‐to‐flower ratios may vary dramatically among years (Copland & Whelan 1989) but are typically very low (Collins & Rebelo 1987; Ayre & Whelan 1989; Hermanutz et al. 1998) and pollen dispersal may be surprisingly limited (Krauss 2000; Richardson et al. 2000; Roberts et al.…”

Are differences in breeding mechanisms and fertility among populations contributing to rarity in Grevillea rhizomatosa (Proteaceae)?

“…Wash and Cas are the furthest apart populations but only c. 9.5 km separates them, and thus it would appear that there is unusual variation in breeding systems within a small area. However, it may be that breeding systems within species of Grevillea are more variable than reported, but because few studies (Richardson et al, 2000;Smith and Gross, 2002) incorporate multiple populations in their assessments of self-compatibility, we possibly have a skewed view of breeding systems variability in Grevillea.…”

“…The amount of viable pollen produced per flower (2953.52 6 1051.29) was low but within the range found for other species of Grevillea (753-15105, N ¼ 5 species, Hermantutz et al, 1998;16136 68.64, N ¼ 20 flowers G. beadleana, Smith, 1997. Most species in Grevillea are self-compatible (see table 1 in Smith and Gross, 2002) with little intraspecies variation in breeding systems (Richardson et al, 2000, but see Hermanutz et al, 1998). The breeding system of G. rhizomatosa, however, varied from self-compatible in the north of the species' distribution (Wash), through to self-incompatible in the center (Dand) and southern limits (Cas) of the species.…”