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Effects of ploidy level on the reproductive biology of tropical Acacia species


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Nghiem, QC 2012 , 'Effects of ploidy level on the reproductive biology of tropical Acacia species', PhD thesis, University of Tasmania.

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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
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 – December)
was slightly earlier than for AA-2x (December – 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.

Item Type: Thesis - PhD
Authors/Creators:Nghiem, QC
Keywords: Acacia mangium, autotetraploid, infertile triploid, invasion, reproductive isolation, pollen-pistil interaction, post-zygotic barriers, embryo abortion, pod/seed production.
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