SUMMARYIn spite of the frequency with which mean performance and sensitivity to macro-environmental variables are found to be highly correlated, there is a growing body of evidence that they are, at least in part under independent genetical control. From a cross between two pure breeding lines which segregate for both of these aspects of the phenotype it should, therefore, be possible to select recombinants which display all four possible combinations of high and low performance with high and low sensitivity. This prediction has been tested by two cycles of selection initiated in the F5 of a cross between varieties 2 and 12 of Nicotiana rustica for the mean performance and environmental sensitivity of two characters, flowering time and final height. Selection was on a family basis, date of sowing being the macro-environmental variable. Qualitatively, three of the four combinations for flowering time and three of the four for final height met the selection criteria by the F5. Quant[tatively, however, all selections fell short of what could have been achieved if all the genetical variation in performance and sensitivity had been independently available for selection. The realised heritability for sensitivity was only half that for performance. The expectation that selection among inbreds produced from the same cross by single seed descent would have been more successful is being tested.
SUMMARYThe possibility of obtaining instant pure-breeding lines by matromorphic seed development in If. rustica has been investigated. Maternal parents homozygous for recessive marker genes and heterozygous for dominant marker genes were pollinated with different .Jficotiana species and with irradiated pollen of a X rust lea variety having a dominant marker. Poilinations with 65 species and with irradiated varietal pollen receiving doses ranging from 8 to 40 kr X-and gamma rays were performed on as many as 40 maternal genotypes.Promising matromorphic progenies were studied for the segregation of marker genes and several quantitative characters and for cytological abnormalities in the first and second generations following their induction, Of the numerous pollinations only one homozygous diploid maternal was recovered from F1 (V5 x Vi) maternal parent pollinated with N. tangsdorffi (2n = 18). Poilinations with 3v. langsdorffi, however, consistently and frequently induced haploids or near haploids. The evidence gathered over many years of experimentations throws serious doubt on the possibility of using matromorphy as an alternative method of producing pure-breeding lines in N. rustica.Transfer of a dominant marker gene from the irradiated pollen to a triple recessive maternal parent has prospective applications in the production of isogenie lines as an alternative to recurrent backcrossing.
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