Grain amaranths (Amaranthus spp.) have been cultivated for thousands of years in Central and South America. Their grains are of high nutritional value, but the low yield needs to be increased by selection of superior genotypes from genetically diverse breeding populations. Amaranths are adapted to harsh conditions and can be cultivated on marginal lands although little is known about their physiology. The development of controlled growing conditions and efficient crossing methods is important for research on and improvement of this ancient crop. Grain amaranth was domesticated in the Americas and is highly self-fertilizing with a large inflorescence consisting of thousands of very small flowers. We evaluated three different crossing methods (open pollination, hot water emasculation and hand emasculation) for their efficiency in amaranth and validated them with genetic markers. We identified cultivation conditions that allow an easy control of flowering time by day length manipulation and achieved flowering times of 4 weeks and generation times of 2 months. All three different crossing methods successfully produced hybrid F1 offspring, but with different success rates. Open pollination had the lowest (10%) and hand emasculation the highest success rate (74%). Hot water emasculation showed an intermediate success rate (26%) with a maximum of 94% success. It is simple to perform and suitable for a more large-scale production of hybrids. We further evaluated 11 single nucleotide polymorphism (SNP) markers and found that they were sufficient to validate all crosses of the genotypes used in this study for intra- and interspecific hybridizations. Despite its very small flowers, crosses in amaranth can be carried out efficiently and evaluated with inexpensive SNP markers. Suitable growth conditions strongly reduce the generation time and allow the control of plant height, flowering time, and seed production. In combination, this enables the rapid production of segregating populations which makes amaranth an attractive model for basic plant research but also facilitates further the improvement of this ancient crop by plant breeding.
BackgroundGrain amaranths (Amaranthus sp) have been cultivated for thousands of years in Central and South America. Their grains are of high nutritional value, but the low yield needs to be increased by selection of superior genotypes from genetically diverse breeding populations. Amaranths are adapted to harsh conditions and can be cultivated on marginal lands although little is known about their physiology. The development of controlled growing conditions and efficient crossing methods is important for research on and improvement of this ancient crop. Grain amaranth was domesticated in the Americas and is highly self-fertilizing with a large inflorescence consisting of thousands of very small flowers. We evaluated three different crossing methods (open pollination, hot water emasculation and hand emasculation) for their efficiency in amaranth and validated them with genetic markers. ResultsWe identified cultivation conditions that allow an easy control of flowering time by manipulating day length manipulation and achieved flowering times of four weeks and generation times of two months. All three different crossing methods successfully produced hybrid F 1 offspring, but with different success rates. Open pollination had the lowest (10%) and hand emasculation the highest success rate (74%). Hot water emasculation showed an intermediate success rate (26%) with a maximum of 94% success. It is simple to perform and suitable for a more large-scale production of hybrids. We further evaluated 11 single nucleotide polymorphism (SNP) markers and found that they were sufficient to validate all crosses of the genotypes used in this study for intra-and interspecific crosses. ConclusionsDespite its very small flowers, crosses in amaranth can be carried out efficiently and evaluated with inexpensive SNP markers. Suitable growth conditions strongly reduce the generation time and allow the control of plant height, flowering time and seed production. In combination, this enables the rapid production of segregating populations which makes amaranth an attractive model for basic plant research but also facilitates further the improvement of this ancient crop by plant breeding.
Populations at expanding edges of species distributions face multiple challenges during colonization. Two important difficulties are lack of mates (Allee effects) and elevated mutational burden (expansion load). Self-fertilization impacts both of these factors: providing mate reassurance and potentially purging recessive deleterious variants. Populations at range margins are often observed as enriched for higher self-fertilization rates suggesting that selfing, may be favored under expansion conditions. Using forward-time, individual-based simulations, we disentangle the effects of range expansion versus selfing to understand the potential benefits from mate reassurance and purging of deleterious alleles. We find that selfers do expand faster, but expansion load still accumulates due to demography, regardless of the mating system. The severity of variants contributing to this load, however, differs across mating system: higher selfing rates allow purging of large-effect recessive variants leaving a burden consisting of small- to intermediate-effect alleles. We test these predictions in the mixed-mating, flowering plantArabis alpina, using whole-genome sequences sampled across an area of both a transition from outcrossing to selfing and from glacial refugia to expanded populations. These empirical results indicate an accumulation of expansion load along with evidence of purging in selfing populations. Our simulations, which disentangle the effects of inbreeding due to range expansion versus due to self-fertilization along with the concordance between both simulated and empirical results suggest that an important beneficial factor for the evolution of selfing during range expansions is the effect of purging, but that purging is not sufficient to prevent load accumulation due to demographic effects.
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