| Specificity of nucleo-cytoplasmic interactions in Triticum
and Aegilops species (a review) S. S. MAAN Professor of Agronomy, North Dakota State University Fargo, North Dakota, 58105, U.S.A. Cytoplasmic differences among related plants are inferred from the different phenotypes of hybrid progenies from reciprocal crosses, because hybrid genotypes are assumed to have differential sensitivity to the parental cytoplasms. Certain hybrid genotypes may not produce noticeable differential phenotypic effects if they share certain nuclear genes which control cytoplasmic effects of both parental cytoplasms. In such cases, parental plants are assumed to have the same or similar cytoplasms, until another nuclear genotype produces differential sensitivity to one or both of these cytoplasms. Cytoplasms of related species are considered the same or similar until they are shown to be different. For example, TSUNEWAKI et al. (1976 and 1978) assumed cytoplasmic similarities between diploid Ae. squarrosa and 5 polyploids with the D-genome, and they based their conclusions on the interactions observed between T. aestivum genomes and the cytoplasms of these Aegilops species. However, interactions involving T. durum genomes indicated cytoplasmic differences among these Aegilops species. Accordingly, Ae. squarrosa was assumed to have contributed the cytoplasm as well as the D-genome to Ae. cylindrica and Ae. ventricosa, and Ae. uniaristata was assumed to have contributed the cytoplasm as well as the M genome to Ae. crassa and Ae. juvenalis (MAAN, 1978). In this case, T. aestivum genomes did not differentiate these cytoplasms and the T. durum genome had differential interactions which separated these Aegilops species into two groups : 1) Ae. squarrosa, Ae. cylindrica, and Ae. ventricosa and 2) Ae. uniaristata. Ae. crassa, and Ae. juvenalis. In general, intraspecific cytoplasmic differences are controlled by one or two nuclear genes. Occasional mutants or off-type genotypes lacking these critical nuclear genes have detrimental cytoplasmic effects and are eliminated immediately or gradually depending on the traits affected and the magnitude of the cytoplasmic effects ; most incompatable or inviable gametes or zygotes, and male-sterile or weak plants may be eliminated each generation and the proportion of less prolific genotypes may be gradually reduced over several generations. Therefore, a species population would be expected to have all of the cytoplasm-specific nuclear genes in homozygous condition. A close linkage between cytoplasm-specific nuclear genes would ensure that (like the cytoplasmic genes) all essential genes will be inherited together. This will contribute to the stability of the nucleocytoplasmic mechanisms and would enhance the fitness of the population. Cytoplasmic and the cytoplasm-specific nuclear gene mutations would persist only if they are followed by specific compensating nuclear gene or cytoplasmic mutations. Certain cytoplasm nonspecific homoeoalleles may partially compensate for the effect of a cytoplasmic mutation, until a complementary nuclear gene mutation has a chance to appear and spread in the population. A mutation with small effect may have a greater chance to survive because a complementary mutation may not be immediately required for the survival of the affected individual. Most mutations producing major phenotypic alterations may be eliminated before a complementary mutation has a chance to appear. Mutations with small phenotypic effects may accumulate in intraspecies populations. The related species populations may differ in the number and kinds of cytoplasmic and cytoplasm-specific nuclear genes, because complementary cytoplasmic and nuclear gene mutations may proceed independently among species. Therefore, interspecific cytoplasmic differences in well-differentiated species are polygenically controlled, and close linkage among these genes may contribute to the development and operation of the interspecific genetic isolating mechanisms. The presence of the species-specific nuclear genes can be inferred only from their sterility or incompatibility interactions with other species-specific genes in interspecific hybrids. Therefore, hybrid progenies from crosses involving related polyploids may be examined to study hybrid sterility components which may include cytoplasmic, sporophytic, chromosomal, or genomic sterilities, because stability and expression of the species-specific nuclear genes may be modified in an alien cytoplasmic and genomic backgrounds. |
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