Recombination rates of soybean varieties from different periods of introduction and release.

Theory predicts that selection for adaptability during the short term also favors selection for a reduced recombination rate in the population. The objective of this study was to test whether the cyclic short-term selection which has taken place in soybean breeding programs in the USA since the introduction of the crop has measurably reduced recombination frequencies. Thirteen soybean varieties separated into four different release periods (prior to 1940, 1940-1954, 1955-1969, after 1970) were evaluated for their recombination frequencies within three locus pairs. Recombination frequencies among the individual varieties ranged from 7.6 to 24.1 % at thep 1 r locus pair, from 20.9 to 30.1 % at thelnp 2 locus pair, and from 28.7 to 41.6% at thedt 1 l 1 locus pair. Recombination frequencies were significantly different among varieties within a release period for thep 1 r andlnp 2 locus pairs, but recombination frequencies did not differ among release periods for any locus pair. Thus, apparently, plant breeders have developed soybean varieties with improved adaptation without influencing recombination rates.