Summary: Polyploid Evolution
|
There are a variety of factors that can influence polyploid establishment (e.g. stochastic variation in populations, environmental and ecological factors, reproductive habit, etc). The minority cytotype exclusion principle is a frequency-dependent process that must be overcome in order for rare polyploids to spread in a (randomly mating) parental diploid population. Most of the progeny that are generated by diploids crossed with tetraploids, will be triploid. Triploids can act as a "bridge" between diploids and tetraploids, and therefore the level of triploid survival and fertility will influence polyploid establishment. Triploids typically have reduced fertility and/or reduced viability, relative to diploids and tetraploids. Two major hypotheses have been proposed to explain this "triploid block". The endosperm imbalance model states that deviation from the endosperm:embryo ratio (3:2) will lead to inviable progeny. Alternatively, the genomic imprinting model (where a gene is expressed differently depending on whether it came from the maternal or the paternal parent) suggests that deviation from the maternal:paternal ratio (2:1 in most angiosperms; or 1:2 in C. angustifolium) in the endosperm could result in abnormal development, and therefore in progeny inviability. The relative fitness levels between cytotypes will influence the magnitude of the threshold frequency required to overcome the minority cytotype disadvantage. In a greenhouse comparison, Burton and Husband (2000) measured six fitness characters in progeny from inter- and intra-cytotypic crosses of Chamerion angustifolium (Onagraceae). Significant cytotype fitness differences were found across all measured life-history stages (Table-1). Survival was similar between diploids, triploids, and tetraploids (~87%). Overall, diploid progeny had the highest fitness (primarily due to a higher seed production). Relative to diploids, the cumulative fitness of tetraploids was 0.67, while that of cross 2 triploids and of cross 3 triploids were 0.12 and 0.06, respectively (or 0.09 for all triploids combined). The endosperm imbalance model is supported, since the overall differences between progeny from crosses 2 and 3 were not statistically different. Tetraploids have a fitness disadvantage relative to diploids, however, tetraploid establishment may be aided by the partial viability and fertility of triploids.
Table-1: Mean fitness ± SE at different life stages in C. angustifolium. (summary of results from Burton and Husband, 2000). *, ^ represent values that are statisically significant from the other values. |
|
Cross (1) 2x x 2x (2) 2x x 4x (3) 4x x 2x (4) 4x x 4x
|
Seed Set 0.63 ± 0.04* 0.32 ± 0.03 0.30 ± 0.04 0.44 ± 0.04
|
Germination 83.40 ± 2.48* 51.00 ± 3.06 68.52 ± 3.71^ 75.29 ± 3.49* ^
|
Biomass 10.66 ± 0.47* 13.57 ± 0.73 13.53 ± 0.43 13.06 ± 0.43
|
Flowering time 54.80 ± 0.66 59.58 ± 6.46* 62.57 ± 8.43* 64.65 ± 8.43^
|
Pollen Production 53.47 ± 3.18 47.00 ± 4.50 28.30 ± 3.97* 55.30 ± 3.97
|
Pollen viability 0.81 ± 0.08^ 0.36 ± 0.08* 0.15 ± 0.02* 0.81 ± 0.04^
|
|
Reference:
|
Get your own Free Home Page