THE EFFECT OF CHROMOSOME NUMBER ON COMPETITIVE ABILITY OF HEXAPLOID WHEAT GAMETOPHYTES

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T w o ltinds of comparatively stable, wheat-like derivatives have been obtained from advanced generation progenies of Triticz~nz Y, Triticale hybrids (O'Mara, 195 1). Strains which have a con~plete complement of wheat chromosomes plus one or more pairs of rye chron~osomes have been designated as alien addition races of wheat. Those strains which are laclting one or more pairs of wheat chromoson~es but which possess a con~pensating number of rye chromoson~e pairs are ltnown as alien substitution races (Gerstel, 194j). Although asynapsis is somewhat more comnlon among the alien chromosomes (01R4ara, 1910), these races are essentially true breeding. Hybrids from crosses between l'~iticu117 ilestim~m and single chron~osorne substitution races have 20 wheat chromosome pairs and two univalents, one being the rye chromosonle and the other the wheat chron~oson~e for which the rye chromoson~e was substituted. The bivalents can be expected to behave normally during meiosis (Sax, 1922). The univalent chromosomes are distributed to the gametes in a random fashion (Longley and Sando, 1930). Thus deficient (n-1) gametes, normal gametes, alien substitution gametes and alien addition ( n f l ) gametes are produced by the hybrid. Other types, such as ganletes \vhich are disomic for the univalent chromosomes, can alsb be expected to occur in rare instances (Ledingham and Thompson, 1938). As the egg is not involved in competition for fertilization, the bacltcross of the wheat x substitution race hybrid to wheat, using the hybrid as the female parent, provides information relative to the frequencies of occurrence of the various ltinds of gametes. The reciprocal backcross gives information concerning the relative efficiency with which the various gametes achieve fertilization. The existence of nullisomic, alien addition and alien substitution races is indicative of the viability of all classes of gametophytes.