Effects of ploidy level on the reproductive biology of tropical Acacia species
Creators: name: Nghiem, QC
Publisher: name: University Of Tasmania
Year: 2023

description: Acacia mangium Willd., A. auriculiformis A. Cunn. ex Benth and their natural hybrid, A. mangium x A. auriculiformis, are important plantation species for timber production in the tropics. However, their potential to invade natural ecosystems has been a concern because of their prolific production of long-lived seeds. Deployment of triploid Acacia clones in plantations might reduce fertility and associated weed risk. Studies were conducted in a hybridizing orchard that was established in southern Vietnam in 2003 with alternate rows of diploid A. mangium (AM-2x), diploid A. auriculiformis (AA-2x) and colchicine-induced autotetraploid A. mangium (AM-4x) clones. Heavy flowering and seed production was obtained for all three species/ploidy combinations; however, the yield of viable open-pollinated triploid (3x) seeds from open pollinated seed had been very low. The reproductive behaviour of the three species/ploidy combinations in the orchard was therefore investigated to determine whether there are barriers to the production of triploid progeny within and between these two species. Peak flowering period for both AM-2x and AM-4x (November ‚Äö- December) was slightly earlier than for AA-2x (December ‚Äö- January). The spikes of AM-4x were shorter than those of AM-2x, resulting in fewer flowers per spike, but they were longer and had more flowers than AA-2x. The proportion of male to hermaphrodite flowers was similar for all three species/ploidy combinations. AM-4x flowers had shorter styles, but the stigma and polyad diameters were greater than those of AM-2x. Differences in stigma and polyad size between cytotypes were not sufficient to adversely affect inter-cytotype pollination. Pollen recovered from the bodies of the main insect pollinators (honeybees) indicated that they did not discriminate in their foraging behaviour. Therefore, neither the floral phenology and morphology of species/ploidy categories nor the pollinator foraging behaviour created barriers to inter-cytotype pollination. Pollen-pistil interactions following different mating combinations within and between each of AM-4x, AM-2x, and AA-2x were investigated. Following controlled pollinations, AM-4x ovules exhibited more attraction to self- than outcross- pollen tubes, in contrast to AM-2x and AA-2x; however, this trend was not consistent for all of the genotypes examined. The reciprocal crosses of AA-2x and AM-2x were successful as pollen tubes grew well in both AM-2x and AA-2x styles and penetrated their ovules. For inter-cytotype crosses, inter-species, particularly those with AA-2x as the maternal parent, had a significantly greater number of ovules penetrated than did intra-species crosses. However, yields of pods and filled seeds following all inter-cytotype crosses were extremely poor, compared to those from the intra-cytotype crosses. Thus, there were strong barriers to production of viable 3x progeny, despite the demonstrated absence of pre-zygotic isolation. Ovule abortion occurred more frequently in all inter-cytotype crosses than in the open-pollinated flowers at 5 and 7 weeks after pollination. Consequently, the proportion of filled seeds set per pod for inter-cytotype crosses was far lower than in pods arising from intra-cytotype crosses and open-pollination. Moreover, the weight of filled seeds from inter-cytotype crosses was significantly lighter than filled seeds from open-pollination, and they were unable to germinate. Analysis using microsatellite markers of DNA obtained from these non-germinated seeds confirmed that most were triploid and had resulted from the target inter-cytotype crosses. It was concluded that abnormal seed development and abortion occurred throughout the 18-week period of pod development, resulting in failure of this set of inter-cytotype crosses to produce any viable triploid progeny. Possible reasons for this are discussed. Research on application of in vitro culture techniques to immature embryos may be required to recover triploid progeny from inter-cytotype crosses of these Acacia species, as has been achieved for other species.