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Apomixis, Polyploidy, and Speciation in Dichanthium

J. M. J. de Wet and J. R. Harlan
Evolution
Vol. 24, No. 2 (Jun., 1970), pp. 270-277
DOI: 10.2307/2406803
Stable URL: http://www.jstor.org/stable/2406803
Page Count: 8
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Apomixis, Polyploidy, and Speciation in Dichanthium
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Abstract

1) Sexual and asexual reproduction are neither genetical nor operational alternatives in Dichanthium, but the overall apomictic mode of reproduction is inherited as a dominant character over sexuality. 2) The agamospecies are in an active state of evolution, and are characterized by several mechanisms that facilitate recombination. 3) They are pseudogamous apomicts, and the sexual as well as asexual embryo sac can develop parthenogenetically, or function sexually after fertilization. 4) The sexual embryo sac usually lies closer to the micropyle than does the asexual sac, and is structurally so modified as to offer a path of least resistance for the pollen tube. 5) Hybridity, and scaling the ploidy level from tetraploid to haploid or hexaploid do not severely effect chromosome pairing; autosyndetic bivalent formation is genetically induced and functional gametes are almost always produced. 6) Haploids obtained from apomictic tetraploids are often fertile and behave cytogenetically like diploids. 7) Fertile haploids produce a large percentage of cytologically unreduced gametes, and when selfed or fertilized by facultative apomictic tetraploids can give rise to fully sexual tetraploids. 8) Diploid races of tetraploid agamospecies are rare, but wherever present, the agamospecies are extremely variable morphologically, suggesting that diploidtetraploid-haploid cycles help apomictic biotypes maintain contact with sexuality. 9) The genotypes of tetraploids are buffered against the shock of absorbing foreign genomes, and extensive introgression often takes place at the tetraploid level between species that are isolated from each other at the diploid level. 10) When the tetraploid taxa are genetically not too distantly related, hybrid haploids are produced that behave cytogenetically like species hybrids, providing a means of gene exchange between diploid taxa that would not cross at this ploidy level. 11) The net result is a dynamic genetic system, capable of a high degree of adaptive polymorphism and progressive evolution.

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