Abstract
BackgroundThe phenomenon of heterosis is critical to plant breeding and agricultural productivity. Heterosis occurs when F1 hybrid offspring display quantitative improvements in traits to levels that do not occur in the parents. Increasing the genome dosage (i.e. ploidy level) of F1 offspring can contribute to heterosis effects. Sugar beet (Beta vulgaris) provides a model for investigating the relative effects of genetic hybridity and genome dosage on heterosis. Sugar beet lines of different ploidy levels were crossed to generate diploid and triploid F1 offspring to investigate the effect of; (1) paternal genome dosage increase on F1 heterosis, and; (2) homozygous versus heterozygous tetraploid male parents on F1 triploid heterosis. A range of traits of agronomic and commercial importance were analyzed for the extent of heterosis effects observed in the F1 offspring.ResultsComparisons of parental lines to diploid (EA, EB) and triploid (EAA, EBB) F1 hybrids for total yield, root yield, and sugar yield indicated that there was no effect of paternal genome dosage increases on heterosis levels, indicating that hybridity is the main contributor to the heterosis levels observed. For all traits measured (apart from seed viability), F1 triploid hybrids derived from heterozygous tetraploid male parents displayed equivalent levels of heterosis as F1 triploid hybrids generated with homozygous tetraploid male parents, suggesting that heterosis gains in F1 triploids do not arise by simply increasing the extent of multi-locus heterozygosity in sugar beet F1 offspring.ConclusionsOverall, our study indicates that; (1) increasing the paternal genome dosage does not enhance heterosis in F1 hybrids, and; (2) increasing multi-locus heterozygosity using highly heterozygous paternal genomes to generate F1 triploid hybrids does not enhance heterosis. Our findings have implications for the design of future F1 hybrid improvement programs for sugar beet.
Highlights
The phenomenon of heterosis is critical to plant breeding and agricultural productivity
By grouping parental germplasm based on regional adaptation, Filial 1 (F1) heterosis increased with increased parental divergence within a range: parents from different parts of the USA when crossed together produced F1 hybrids with considerable heterosis, but when parents from the USA were crossed with Mexican varieties, which represents a wider cross, there was less heterosis [20]
F1 diploid hybrids of sugar beet exhibit positive heterosis effects on seed traits To determine the extent of heterosis effects on sugar beet seed traits at the diploid level, the viability and size of sugar beet seeds of the parental and F1 generations were analyzed (Table 1 and Table 2)
Summary
The phenomenon of heterosis is critical to plant breeding and agricultural productivity. Sugar beet lines of different ploidy levels were crossed to generate diploid and triploid F1 offspring to investigate the effect of; (1) paternal genome dosage increase on F1 heterosis, and; (2) homozygous versus heterozygous tetraploid male parents on F1 triploid heterosis. More recent experiments utilizing molecular markers have found no correlation between parental genetic distance and heterosis in maize [21,22,23] Other crops where this has been investigated (e.g., bread wheat, rice, pepper, oilseed rape) and in models such as Arabidopsis thaliana, have shown limited or no evidence supporting this approach to selecting parental lines for triggering heterosis effects in F1 hybrids [24,25,26,27,28,29,30,31]
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