EVIDENCE FOR GENETIC RESTRICTION OF RECOMBINATION IN THE LIMA BEAN

Abstract

is some evidence from studies of related species of plants that low T E a E a frequency is associated with outcrossing and high chiasma frequency is associated with selfing (reviews in GRANT 1958; BODMER and PARSONS 1962). From this it has been inferred that crossover rates have been adjusted by natural selection toward high values in selfing species to compensate for the restrictions imposed on the variability system by breeding habit, and vice versa for outcross-ing species. The present experiment on the lima bean, a highy self-pollinated species, is reported for the bearing it has on the adjustment of mechanisms which regulate recombination rates. Estimates of recombination between any given pair of linked loci in lima beans often differ strikingly in different hybrids and/or in different environments (ALLARD 1956; ALLARD and CLEMENT 1959). Similar fluctuations have long been known to occur in other plant species, e.g. maize (STADLER 1925). It was therefore decided to divide the experiment into two parts. In the first part, genetically uniform hybrid plants were sown, or harvested, at different times to determine the effect of various natural environments on recombination values. In the second part of the experiment, selection for high and low recombination values was practiced among selfed progeny of certain hybrids to detemine the extent of chromosomal control of recombination values. MATERIALS A N D METHODS Three different pairs of linked loci were studied, as shown in Table 1. Pheno-typic expressions of these loci are: R/r-dark red us. red seed-coat color; Wl/wl-lanceolate us. ovate-lanceolate us. ovate leaflet shape; D/d-indeterminate us. determinate habit of growth; L,/l,-green us. yellow plant color; S/s-mottled us. diffuse-mottled us. self-colored seed coats. These particular pairs of loci were selected for study because classification into alternative phenotypes is easy and certain. Also the three combinations R Wl/rwl, SL,/sl, and WlD/wld represent loose, moderate, and close linkage, respectively. In addition, one gene pair in each combination is partially dominant, leading to six-class segregations and hence to greater efficiency in estimating recombination values than is possible with less complete classification. Use of multiple segregating stocks would have been desirable to test whether recombination values respond independently or jointly to changes in environment, or to selection, but unfortunately suitable stocks were