The first and sixth generations of a randomly mated, autotetraploid red clover (Trifolium pratense L.) population (2n=28) derived by treatment with nitrous oxide were examined to determine changes in male and female fertility. Characters measured were chromosome pairing, pollen stainability, and seed set. Approximately equal numbers of aneuplolds (2n−1, 2n+1, 2n+2) were observed in each generation tested. These aneuploids produced less stainable pollen and seed than did euploids in greenhouse crosses, but under field cage conditions, only 2n=27 genotypes produced fewer seeds per head than the euploid genotypes (2n=28). However, clones with 27, 28, and 30 chromosomes produced lighter weight seeds than clones with 29 chromosomes. It was concluded that six generations of random mating were ineffective in changing meiotic configurations, pollen stainability, and seed yield. Although not directly compared in this study, aneuploids appeared to be produced in about the same frequency in nitrous‐oxide and colchicine‐derived tetraploid populations. Aneuploids apparently are not a major factor in the low seed yields of tetraploid red clover. It is suggested that direct selection for high seed yield may be more effective than selection for absence of aneuploids.
Chromosome numbers for eight Trifolium species were determined. These represented first counts for T . philisticum Zoh. and T . strepens Crantz. and confirmation for T . cmmabile H.B.K., T . bads'um Schreb., T . megalanthum Steud., T . virginicum Small., T .aureum Poll., and T . physodes Stev. ex M . B . Six of the eight species had 2n = 16 chromosomes. Trifolium megalanthum, T . badium, T . aureum and T . strepens were found to have 2n = 32, 2n = 14, 2n = 16, and 2n = 16 chromosomes, respectively.
Crosses were made between 2X and 4X red clover (Trifolium pratense L.) plants under field cages by honey bees (Apis mellifera L.) to investigate the feasibility of producing autotetraploids by the mechanism of unreduced gametes. Clones from three cultivars and one experimental strain were used as female parents (diploids) and plants from a nitrous oxide‐derived tetraploid population of the cultivar ‘Kenstar’ were used as the male parents. Diploid parents yielding low amounts of seed produced a high percentage of tetraploids, but those yielding large amounts of seed produced very few tetraploids. High seed set apparently via pseudo‐self‐compatibility or 2X‐2X crosses from pollen contamination, interfered with the production of seeds via unreduced gametes in some cages. The frequency of tetraploids derived from diploid female parents in other cages where monoploid pollen was apparently excluded ranged from one per 1500 to 11000 flowers. About half of the apparent tetraploids were aneuploids that could not be distinguished from euploids except by chromosome examination. The frequency of univalents, bivalents, and multivalents at metaphase I of the unreduced gamete population was similar to that of the nitrous oxide‐derived population. In comparison to the colchicine‐derived material, there were more univalents and bivalents and fewer multivalents at metaphase I of the unreduced gamete population. Further research on the exact mechanism of nitrous oxide doubling and a comparison of the three methods is necessary.
Isolation barriers were examined by making all possible crosses including reciprocals among arrowleaf clover (Trifolium vesiculosum Savi), T. mutabile Portenschl. and T. leiocalycinum Boiss. & Spruner. No evidence of sterility was indicated by cross seed set. All F1 cross seed germinated readily but yellow seedlings were produced by crosses of T. mutabile and T. leiocalycinum with T. vesiculosum. F1 plants from reciprocal crosses of these species were normal green. The yellow condition disappeared as plants matured. Most F1 plants from crosses between accessions produced less stainable pollen than plants from crosses within accessions. All species had 16 somatic chromosomes and chromosome associations at metaphase I of meiosis were similar for all parents and F1 progenies. We concluded that T. mutabile and T. leiocalycinum are quite closely related and are slightly differentiated from T. vesiculosum. Therefore, the three taxa must be isolated under field conditions to prevent crossing.
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