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

Abstract

Plant breeding systems are seldom studied across the breadth of a species' range. For many systems, this precludes an informed assessment of the evolutionary biology of a species, particularly of the factors that shape fecundity. 'Grevillea rhizomatosa' is a threatened species of shrub known only from a 7 x 8 km area and c. 2000 plants in northern New South Wales, Australia. The species reproduces asexually from rhizomatous suckers, and fruit are only produced in a few populations. Over two flowering seasons, we investigated the extent of sexual reproduction and the mechanisms of infertility in five populations that span the range of the species. Seed were produced in three of the five populations. The breeding system varied among populations from obligate outcrossing to facultative outcrossing to fully sterile. Fruit to flower ratios were below 0.13 in the fertile populations but within the range found in other species of Grevillea. Pollinator limitation was not linked to infertility. Stigmatic opening and receptivity were functional in all populations. Interpopulation crosses using fertile pollen failed to recover fertility in an infertile population. A breakdown in female and male fertility mechanisms was found including a fault in the mechanical release of pollen from anthers, <10% viable pollen, and a post-pollen-deposition event that prevents fertile pollen from effecting seed-set. In the infertile populations pollen was not released from anthers, resulting in flowers projecting barren pollen-presenters. Sexually reproducing populations are threatened by the incursion of asexual forms that may be favored by frequent disturbance from wild-fires.