Will polyploidy provide a reproductive advantage in a changing climate? A test case with Themeda triandra (Kangaroo grass)

Honours project: Amelia Stevens

Seed traits, including dormancy and germination, and subsequent seedling establishment and growth, play an important role in determining plant population dynamics.

While it is increasingly demonstrated that within-species variation in seed and seedling traits is often associated with environmental factors, polyploidy is another significant factor that may influence variation.

Polyploidy, also referred to as whole genome duplication, is the heritable condition of possessing more than two sets of chromosomes. It is common in flowering plants and is often associated with wider ecological tolerances.

I hypothesised that interactions between polyploidy and the seed development environment would affect dormancy, germination and early growth, and that tetraploids would exhibit seed and seedling characteristics that suggest greater fitness in more variable and extreme climates.

I investigated the widespread, native grass Themeda triandra (a known polyploid complex) which is commonly used in restoration. I set out to determine how diploids (2n = 20) and tetraploids (2n = 40) differ in early life traits from seeds matured under two different seed development environments (warmed and ambient).

Using seed and seedlings from over 60 maternal lines I investigated cytology, dormancy, germination and seedling growth. Flow cytometry confirmed the cytotypes of offspring were the same as the maternal plant and revealed that cross-pollination between cytotypes is unlikely.

By conducting dormancy alleviation experiments I found, for the first time, tetraploids to be significantly more dormant than diploids, regardless of environmental factors. Germination experiments using drought stress and two temperature conditions revealed non-dormant tetraploids to be more sensitive to germination stress compared to non-dormant diploids.

Finally, measurement of seedling growth found tetraploids have larger and faster growing seedlings compared to diploids, and unlike diploids, tetraploids maintained a similar seedling size regardless of seed developmental warming. Seed and seedling traits of tetraploids appear to be better adapted to stressful environments than diploids since the seeds are more dormant and more selective about when they will germinate; and when they do germinate seedling growth is not affected by the previous maternal environment.

These results provide some of the first experimental research addressing the role that polyploidy plays in governing seed and seedling traits and tolerance of novel or stressful environments in a keystone native species. These results can inform seed sourcing and use in restoration and conservation in a rapidly changing climate and have improved our understanding of the genetic basis of climate adaptation in T. triandra

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