In a broad sense gametophytic selection (GS) can result from pollination competition as well as from post-pollination competition. Post-pollination male gametophytic fitness depends mainly on pollen germination time, tube growth rate and fertilization ability.
Several data recently produced indicate that GS can produce significant evolutionary changes and can be used to develop efficient methods of plant breeding. In fact the phenomenon is assumed to rely on two main factors: i) genetic variability of gametophytic origin and ii) gametophytic-sporophytic gene expression. Besides the classical examples of wx, Ae and Adh, information supporting that these assumptions are valid for a large portion of the genome has been obtained by different authors. The data concern male gametophytic selection of several chromosomal deletions, maize pollen tube elongation in vitro, tomato and maize isoenzyme patterns in pollen and in sporophytic tissues and mRNA, from pollen and shoots of Tradescantia and maize.
Selection for pollen competitive ability in maize produced positive and correlated response for sporophytic traits (Ottaviano et al., TA.G. 63:249-254). However, in view of the selection procedure adopted, the results furnished only an indication that selection response is due to the variability of genes expressed in the gametophytic phase. For this reason a more comprehensive experiment strictly based on within-plant gametophytic selection has been carried out. The selection criterion derives directly from the maize ear structure: the silk length varies according to the position of the flower on the ear, increasing from the top to the base. Within-plant selection is applied when the pollen from the same heterozygous plant is used to pollinate a single plant, either for selfing or for crossing. Differences in the progeny due to the position of the kernels on the ear (apex or base) reveal response to gametophytic selection due to genes expressed in the postmeiotic male gametophytic phase. The selection procedure adopted is a recurrent selection scheme, where the sporophytes (plants) were chosen strictly at random. After two cycles of selection two populations were produced: Base population (high gametophytic selection intensity) and Apex population (low gametophytic selection intensity). Response to selection was evaluated in 60 S2 and 160 FS (full-sib) families derived from the two populations. The S2's (30 base and 30 apex) were used to study pollen competitive ability and the FS's (80 base and 80 apex) to evaluate the correlated response of sporophytic traits: 50 kernel weight (50-KW), kernel number per row (KNR) and number of kernel rows per ear (RN).
Gametophytic competitive ability of each S2 family was evaluated in comparison with a standard inbred line by means of a mixed pollination technique. The value is expressed as the coefficient of regression (b) describing the variation of uncoloured kernels (the standard produces coloured aleurone) from the apex to the base of the ear.
The mean value of gametophytic competitive ability in the progeny produced at higher selection intensity is higher than that of the progeny produced at low selection intensity (b = 0.31 and -1.24 for the base and apex population respectively; the difference is statistically significant, P<0.05), showing that the variability of the character is largely based on genes expressed in the gametophytic phase. Sporophytic effects could also play an important role; however, considering the selection procedure used, they should not contribute to the selection response obtained.
A positive correlated response is observed for mean kernel weight (50KW = 15.67 and 14.53; a significant difference, P<0.05), a character which reflects growth in the endosperm. KNR and RN, which are mainly related to developmental processes, do not reveal significant differences. Considering that the base population was intercrossed for several generations and therefore linkage should not play an important role in the correlated response, the results obtained indicate that there are genes controlling basic physiological processes of growth which show sporophytic-gametophytic genetic overlap.
E. Ottaviano, M. Villa and A. Legrenzi
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