Inbreeding in two maize subpopulations selected for tassel size

Abstract

The present work was based on the evaluation of full sib and S 1 progenies from the maize (Zea mays L.) population ESALQ-PB1 after six cycles of divergent selection for tassel size. The following traits were analyzed: plant height , ear height, tassel branch number, tassel length, and tassel weight. For all traits, the experimental units were means of three plants per plot. Progenies were evaluated in eleven experiments (completely randomized blocks) with three replications in Piracicaba -SP, Brazil. Means of inbred (m 1 ) and non-inbred (m 0 ) progenies were used to estimate inbreeding depression (I = m 1 -m 0 ) and the contribution of homozygotes (µ 0 + a*) and heterozygotes (d*) to the population mean. Considering the five traits under study, inbreeding depression varied from 1.9 to 15.9% but significance for that effect was detected only for plant height. Plant traits exhibited higher depression than tassel traits; and tassel branch number seems to be more sensitive to inbreeding than the other two tassel traits. Except for plant height, it was apparent that inbreeding depression was higher in the subpopulation negatively selected for tassel size (T-). Estimates of A = µ 0 + a* and d* indicate a smaller effect of dominance deviations for all traits, as compared to the contribution of homozygotes. Significant variability among progenies was detected in most instances. Key words: inbreeding depression, divergent selection, tassel characters ENDOGAMIA EM DUAS SUBPOPULAÇÕES DE MILHO SELECIONADAS PARA TAMANHO DO PENDÃORESUMO: O presente trabalho foi baseado na avaliação de progênies de irmãos germanos e S 1 da população de milho (Zea mays L.) ESALQ-PB1 depois de seis ciclos de seleção divergente para tamanho do pendão. Os seguintes caracteres foram analisados: altura da planta, altura da espiga, número de ramificações do pendão, comprimento do pendão e peso do pendão. Para todos os caracteres, as unidades experimentais foram médias de três plantas por parcela. As progênies foram avaliadas em onze experimentos (blocos completamente casualizados) com três repetições em Piracicaba -SP, Brasil. As médias das progênies endogâmicas (m 1 ) e não endogâmicas (m 0 ) foram usadas para estimar a depressão por endogamia (I = m 1 -m 0 ) e a contribuição de homozigotos (µ 0 + a*) e heterozigotos (d*) para a média populacional. Considerando os cinco caracteres sob estudo, a depressão por endogamia variou de 1,9 to 15,9% mas somente para altura da planta foi detectada significância para aquele efeito. Os caracteres da planta exibiram maior depressão do que os caracteres do pendão; e o número de ramificações do pendão parece ser mais sensível à endogamia do que os outros caracteres do pendão. Com exceção de altura da planta, foi evidente que a depressão por endogamia foi maior na subpopulação selecionada negativamente para tamanho do pendão (T-). As estimativas de A = µ 0 + a* e d* indicam um menor efeito dos desvios de dominância para todos os caracteres, quando comparados com a contribuição dos homozigotos. Foi detect...

Inbreeding depression in single, three way and double-cross hybrids of maize

“…In the study of Farias Neto and Miranda Filho (2000), for example, the values of inbreeding depression were -10.1 and -11.3%. Maldonado and Miranda Filho (2002) evaluated two generations in three populations and obtained values from -14 to -4.1%.…”

“…It should be emphasized that the commercial hybrids stood out in relation to the inbred generation (Table 2). Farias Neto and Miranda Filho (2000) showed in their study that plants coming from S 0 seeds (originating from self-pollination) exhibit greater depression in characteristics controlled by greater levels of dominance. As the maize plant is an allogamous species, it is favored by the genetic diversity of its parents; thus, the heterosis levels rise in accordance with greater diversity.…”

“…For the first trait, the contribution from the loci in heterozygosity ranged from 0.08 m for the cultivar 30F90Y, to 0.16 m for the hybrid AG7088. Farias Neto and Miranda Filho (2000) obtained a value of 0.62 m for the plant height trait and 0.56 m for loci in heterozygosity. Maldonado and Miranda Filho (2002) estimated values that ranged from 0.112 to 0.758 m for the same trait.…”

Inbreeding depression in maize populations of reduced size

“…Other results of inbreeding depression for yield in S 1 progenies have shown average decreases varying from 37.5% to 64.0% (Vianna et al, 1982;Gama et al 1985;Naspolini Filho & Vencovsky, 1982;Moro, 1982;Mota, 1984;Miranda Filho & Meirelles, 1986;Marques, 1988;Terasawa Jr., 1983;Nass & Miranda Filho, 1995;Packer, 1998). Farias Neto & Miranda Filho (2000) also reported on inbreeding depression for PH, EH and tassel traits in subpopulations divergently selected for tassel size. The low inbreeding depression expected for 50% homozygosity can be due to the low exposure of the potential genetic load as a consequence of low levels of homozygosity resulting from low inbreeding in populations of reduced size.…”

Variability and inbreeding in semiexotic maize populations

“…The same point had been already reinforced by Miranda Filho and Viégas (1987). Farias Neto and Miranda Filho (2000) reported on inbreeding depression for plant and ear height and for tassel traits (weight, length, and branch number) in two subpopulations divergently selected for tassel size; except for plant height, depression rates were higher for the subpopulation selected for smaller tassel size. For yield traits (ear weight or grain weight), several reports observed inbreeding depression in the range of 37.5 to 68.0% (Mota, 1974;Vianna et al, 1982;Gama et al, 1985;Miranda Filho and Meirelles, 1988;Nass and Miranda Filho, 1995;Packer, 1998;Simon et al, 2004).…”