Exotic germ plasm of Arachis L. interspecific hybrids

Abstract

One thousand seventy-five cross combinations are re ported among 91 collections of the genus Arachis arising from northeastern Brazil to northwestern Argentina and from the south coast of Uruguay to the north-western Mato Grosso. The region sampled covers a land area about the size of the United States (excluding Alaska) between latitudes 3°-35°S and longitudes 35°-66°W south of the Amazon and from the base of the Andes to the Atlantic. The collections were distributed among the seven sections of the genus as follows: 22 Arachis (A), 26 Erectoides (E), 31 Rhizomatosae (R), 2 Caulorhizae (C), 6 Extranervosae (EX), 2 Triserninalae (T), and 2 Ambinervosae (AM). Cross-combinations were attempted between species from all of the generic sections. The number of combinations less 121 repeats represents 11.6 percent of the total number of paired combinations possible (8190) among the 91 collections. Two hundred ninety-six of the cross-combinations resulted in hybrids, 779 failed to pro duce hybrid offspring. Counting a hybrid and its reciprocal to be one hybrid, at least 126 of the 296 obtained were hybrids between distinct species. Among these are eight interspecific hybrids with the cultivated peanut, all con fined to section Arachis to which A. Iiypogaea belongs. In no instance following 69 trial crosses, with species representing all other sections, was a single intersectional hybrid obtained with A. hypogaea. Among the 2n × 2n hybrids and 4n × 4n hybrids, where numerical balance of the chromosomes was not a factor, the mean fertility of the within-section hybrids as represented by percent pollen stained was: A × A, 30.2 percent; E × E, 12.9 percent; R2 × R2 68.1 percent; C × C, 86.8 percent; EX × EX, 0.2 percent; T × T, 59.5 percent; and AM × AM, 20.6 percent. All intersectional hybrids av eraged 1.9 percent pollen stained. The data were used in combination with those of geographic distribution and presumed paleobotanical history to imagine the migration pattern and sectional evolution of the genus. The data supported the concept of a genus composed of three old centrally located botanical sections surrounded by four more recently derived sections to one of which (section Arachis) the cultivated peanut belongs. Intra- and intersectional cross-compatibilities and pollen fertilities were related to the problem of breaking the barriers to genetic exchange between species and to the problem of using genetic information from other species for the genetic improvement of the cultigen.