Inheritance of the Albinistic Non-Conidial Characters in Interspecific Hybrids in Neurospora

Abstract

Summary Further experiments in crossing typical conidia 1 races of the Monilia bread mold, Neurospora sitophila, with albinistic non- conidial races of the species indicate that asci from such a mating are heterozygous for conidia, and that segregations of the factors for sex and the factors for conidia occur independently. Both sets may segregate in the first nuclear division in the ascus, both sets in the second division, or one set in the first and one set in the second. The eight spores may thus alternate in the ascus: two and two; four and four; or two, four, two, as to their sex or the factors for conidia which they contain. The albinistic non-conidial race of N. sitophila was crossed with typical conidial N. crassa and fertile hybrids were obtained. The F₁ asci usually mature only two or three spores. Of the forty-four haplont clons studied, only six were strictly non-conidial Five were albinistic intermediates producing a few conidia abnormally. Thirty-three were conidial, resembling the crassa parent in coloration. The albinistic non-conidial f₁ strains were back-crossed with the crassa parent. The hybrids obtained were very fertile, with little abortion of the ascospores. The asci and spores resemble more those of the crassa parent. The few asci analyzed showed that in some cases the segregation of the factors for conidia was completed either in the first or the second nuclear division, so that the spores are disposed two and two; four and four; or two, four, two; as was proved to be the case for intraspecific crosses in N. sitophila. Some evidence was obtained indicating that in these back-cross asci segregation of the factors for conidia is occasionally not perfect, resulting in intermediate types with albinistic aërial growth, but with a few conidia rather abnormally formed. Hybrids were obtained by crossing the albinistic non-conidial race of N. sitophila, which has 8-spored asci, with a weakly conidial, “scanty growth,” 4-spored segregate 507, resembling N. tetrasperma as to its asci. The F₁ asci were 8-spored, but with much spore abortion after delimitation. Over 60 per cent of the f₁ haplonts grown from ascospores chosen at random were non- conidial. Some of these were fluffy albinistic types; others produced only a scanty aërial growth. Some haplonts produced highly colored conidia in far greater abundance than does the conidial 4-spored parent. When the non-conidial f₁ haplonts were crossed with S₆, pure N. tetrasperma, the asci developed in the fruit bodies from this mating contained variable numbers of spores, mostly from three to six. Mycelia obtained by growing the small unisexual spores showed still greater variation in the color and abundance of conidia produced. Again over 60 per cent of such haplonts did not produce conidia. Non-conidial segregate haplonts were again crossed with N. tetrasperma with the result that now many asci with four spores were produced. Spores from several asci were isolated in order and germinated. A number of the hermaphroditic mycelia obtained showed either weak sexuality or incompatibilities, some even approaching neutrals; otherwise segregations of the factors for sex and for conidia appeared to be Mendelian. Both nuclei in a bisexual spore from these hybrid asci may carry +C factors for conidia; one of the nuclei may be conidial and the other not; or both nuclei may be non-conidial, depending on the way the segregations of factors for sex and for conidia take place and the redistribution of the nuclei before the spores are cut out. When both nuclei carry the +C factors, the mycelium from that spore will develop three kinds of conidia as regards their sex. If only one of the two nuclei in a bisexual spore carries the +C factors, then only one kind of conidia as to sex will be cut off. Hermaphroditic, totipotent races were crossed with strictly unisexual races, and fertile hybrids were obtained as the result of a preference for cross-fertilization over selfing, the ascocarps forming along the line in each case where the two mycelia met in plate cultures. It was proved also that lines of ascocarps may be formed across a plate culture in which two such mycelia are grown, without any act of cross-fertilization entering into the phenomenon. Some hermaphroditic races, such as pure tetrasperma and 5M', which produce ascocarps freely on their own mycelia, when grown opposite each other in plate culture do not form lines of fruit bodies where the two mycelia meet. When certain other hermaphroditic strains, however, are grown opposite these races, such lines of ascocarps are formed where the two mycelia meet, and these fruit bodies are in addition to the ones produced by each mycelium over the area occupied by itself alone.