Inbreeding depression in maize populations of reduced size

Abstract

Inbreeding is a well known phenomenon in living beings and its immediate consequence is the decrease in the expression of quantitative traits, known as inbreeding depression. Selfing is the most common system of inbreeding in plant species; however, little has been studied with other less severe inbreeding systems, such that resulting from small population sizes. The present work consisted of the study of the inbreeding effect on quantitative traits as a consequence of reduced population size under panmixy. Three maize (Zea mays L.) populations were used in this study: P1 -- ITA, population derived from the variety IAC-Taiúba; P2 -- represented by 30 subpopulations already submmited to reduced size (N = 5); and P3 - population derived from the interpopulation cross ESALQ-PB2 x ESALQ-PB3. The subpopulations and the respective parental populations were evaluated in six experiments using completely randomized blocks with four replications in Piracicaba (SP) and Anhembi (SP), Brazil, from 1997 to 1999. Estimates of inbreeding depression and components of means were obtained for the two generations in the three populations for the following traits: plant height, ear height, ear length, ear diameter, and yield traits (total ear weight and total grain weight). In all populations and for all traits and sampling generations, means of subpopulations were always smaller than mean of the base populations, however the inbreeding depression levels were smaller than expected. The highest inbreeding depression was exhibited by the yield traits, while a very small depressive effect was observed for plant height and ear height in the first generation of reduced size in populations P1 and P3. The component A (expected mean of a random sample of completely homozygous lines) was always higher than d (contribution of the heterozygotes to the mean) for all traits and populations.

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

“…In the study of Maldonado and Miranda Filho (2002), inbreeding depression for the plant height trait was from 1.0 to 12%, very near the values we obtained, which varied from 2.40 to 8.00% for the cultivars 30F90Y and Balu761, respectively. Inbreeding depression in the trait of distance from the soil surface to the base of the first ear was also low, ranging from -6.38% for 30F90Y to 10.87% for Balu761.…”

“…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.…”

“…Other studies estimated inbreeding depression for the ear weight trait and found values that were not very consistent, values much lower or intermediate in relation to the values found in this study. Studies such as those of Maldonado and Miranda Filho (2002) with maize showed that inbreeding depression in the plant height and ear height traits is not expressed as much as in grain yield. This is due to greater genetic complexity, showing that such a characteristic does not have complex dominance as much as characteristics inherent to yield.…”

“…Maldonado and Miranda Filho (2002) evaluated two generations in three populations and obtained values from -14 to -4.1%.…”

Variability and inbreeding in semiexotic maize populations

“…Lima et al (1984) studied 32 Brazilian populations with 50% expected homozygosity and found inbreeding depression varying from 27.0 to 57.9% for grain yield, 7.5 to 20.3% for plant height and 6.9 to 27.7% for ear height. Maldonado and Miranda Filho (2002) studied the effect of inbreeding in subpopulations of small effective size in four generations (F = 0.125, 0.234, 0.291, and 0.379; F is the Wright's coefficient of inbreeding in the sequence of generations) of three populations; for grain yield, the inbreeding depression adjusted to 50% homozygosity (expected level for S 1 progenies) varied from 18.1 to 24.8% when considering all populations and generations, which were below the results reported by Lima et al (1984), probably because the lower exposure of deleterious recessive genes of large effects, under a less severe inbreeding system.…”