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A Cytological Monograph of the American Species of Tradescantia

Edgar Anderson and Karl Sax
Botanical Gazette
Vol. 97, No. 3 (Mar., 1936), pp. 433-476
Stable URL: http://www.jstor.org/stable/2471708
Page Count: 44
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A Cytological Monograph of the American Species of Tradescantia
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Abstract

1. Autopolyploidy, aneuploidy, segmental interchange, fragmentation, and differences in chromosome size were found in the nine genera and thirty-two species of the Commelinaceae which were investigated cytologically. 2. Chromosome pairing has been studied in five diploid species of Tradescantia. The average number of chiasmata per bivalent is found to be fairly constant for any one plant but varies widely from plant to plant, even within the same species. 3. Pollen sterility varies from 82 to 98 per cent among the diploid species and ranges from 71 to 89 per cent among the autotetraploids. 4. An extensive cytological survey of the species allied to T. virginiana shows them to be made up largely of (1) diploid races and species, mostly southern, and (2) autotetraploid races and species, mostly northern in distribution. 5. The autotetraploids have roughly twice the average radius of distribution of the diploids. 6. Terminal chiasmata and median attachment constrictions, conditions favoring autotetraploidy, are discussed. 7. Autotetraploid Tradescantias are slightly larger and have longer blooming periods than the related diploids, but can be distinguished from them only cytologically. 8. Cytological conditions favoring autotetraploidy have brought about the independent evolution in different species of Tradescantia of vigorous autotetraploid races. 9. Five artificial interspecific diploid hybrids were produced experimentally and studied cytologically. Pairing in these hybrids is regular although the chiasma frequency is slightly lower than the average of the parents. Pollen fertility varies from 40 to 73 per cent. The evidence therefore indicates that structural changes have played little or no part in the immediate differentiation of these species. 10. Natural hybrids are discussed and illustrated by examples. 11. Certain of the internal and external barriers to interspecific hybridization in the American Tradescantias are enumerated. 12. The various consequences of hybridization under different conditions are described. 13. The cultivated Tradescantias are compared with the wild species from which they were ultimately derived and their cytological differences are compared statistically.

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