Heterochrony and inbreeding
--Abedon, BG and Tracy, WF
We have observed alteration in the timing of the juvenile and/or adult-vegetative phases as a result of recurrent selection for agronomic traits in a number of maize populations (Abedon and Tracy, p. 70 in Abstr. 37th Annu. Maize Genet. Conf., 1995). Correlated responses to selection may be caused by a number of factors including pleiotropy, inbreeding, linkage, and genetic drift. Our objective was to determine the effects of inbreeding on several morphological traits that are used as markers of the timing of juvenile and adult-vegetative phases in order to better interpret results from our recurrent selection studies.
Populations with different levels of inbreeding were generated by selfing 20 plants from the sugary1 population Minn11P c3, which had previously undergone three cycles of recurrent selection for pseudostarchiness. For this experiment, seed from individual plants was mixed in a balanced bulk for each generation of inbreeding to form populations S1, S2, S3, S4, and S5. These five populations plus the original population (S0) were grown in 1995 in randomized complete blocks over two planting dates (15 May and 13 June) with four replications per planting date. Three-row plots were overplanted and thinned to 15 plants per row. Data were collected on ten plants from the middle row of each plot. The following developmental traits were evaluated: first leaf with adult wax, last leaf with juvenile wax, last node with adventitious roots, tiller number, first leaf with pubescence, and ear leaf and total leaf number. First leaf with adult wax was evaluated only in the early planting date. Several traits were also examined that are known to exhibit inbreeding depression, including: leaf length, leaf width, days to 50% anthesis and silking, ear height, and plant height. Data were analyzed by analysis of variance and LSD (p<0.05) was used for means comparisons.
Inbreeding depression was evident for all traits known to respond to inbreeding. Leaf length, leaf width, ear height, and plant height decreased significantly, and flowering time was significantly later, between the S0 and S5 populations (Table 1). Of the traits associated with phase change, only tiller number and total leaf number decreased significantly between the S0 and S5 populations (Table 2). Ear leaf number and most developmental traits associated with the timing of vegetative phases (first leaf with adult wax, last leaf with juvenile wax, first leaf with pubescence, and last node with adventitious roots) were unaffected by inbreeding with no significant differences between most populations, particularly S0 and S5 (Table 2).
Table 1. Agronomic trait means at six levels of inbreeding (S0-S5),
pooled over blocks and planting dates.
|Inbreeding generation||Leaf length (cm)||Leaf width (cm)||Ear height (cm)||Plant height (cm)||Days to 50% anthesis||Days to 50% silking|
Table 2. Developmental trait means at six levels of inbreeding (S0-S5),
pooled over blocks and planting dates.
|Inbreeding generation||First adult wax||Last juvenile wax||First leaf with hairs||Tiller #||Last node with adv. roots||Leaves below ear||Total leaf #|
These results indicate that most morphological markers of the juvenile (last leaf with juvenile wax, last node with adventitious roots) and adult (first leaf with adult wax, first leaf with pubescence) vegetative phases are not affected by inbreeding depression, suggesting that these traits are governed primarily by additive gene action. Tiller number, which has been used as a marker of the juvenile-vegetative phase in studies of heterochronic mutants, was significantly affected by inbreeding depression, suggesting that dominant gene action governs this trait. Tiller number may not be a useful heterochronic marker in wild type populations of maize. In a diallel of six maize populations, Revilla et al. (p. 84 in Abstr. 87th ASA Meeting, 1995) found a significant (p<0.05) correlation among last leaf with juvenile wax, first leaf with adult wax, and last node with adventitious roots (last leaf with pubescence was not evaluated in that study), but no correlation between any of these traits and tiller number. Together, these results suggest that the timing of vegetative phase change in Minn11P c3 is governed primarily by additive gene action, although a dominance component may exist. This agrees with Revilla et al. who found significant (p<0.05) general combining ability for these same traits while specific combining ability was not significant.
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