Helianthus annuus (Common Sunflower) Arizona, 2015
I have seen sunflowers in glorious rows in fields in Australia, Europe, and the USA, but in Arizona I got to see the original wild sunflower. Well, maybe... Sunflowers are large daisies that have been bred and grown by humans for their nutritious seeds for at least 4,000 years (Blackman et. al., 2013) and more recently for their potential as biofuel alternatives. Originally native to Southern and central USA and Northern/Central Mexico, they are now cultivated across the world, and are grown as specially bred crop plants across the US, including in areas that were home to their original ancestors. Subsequently many plants have escaped from crop populations on numerous occasions making it hard to work out whether the populations you see along roadsides in Arizona, or other southern states, are natural populations still persisting, plants that have jumped the fence from nearby farms, or natural populations that have been interbreeding with domesticated sisters in the next field (Kane & Rieseberg, 2008). Sunflowers (of which there are about 70 species) are famous for this ability to share genes between species and for forming hybrids, populations that are part species A and part species B. Ongoing hybridization between otherwise distinct species can be an advantage where genes that evolve in and are good for one species, can be acquired by another species where they are also advantageous (e.g one species happens upon a gene that improves drought tolerance, a gene that would be of use to species B if it could capture it). Similarly, there are interesting, if relatively rare, cases where hybrids may find a niche of their own in which they grow better than either parent plant. This has been found to be the case with Helianthus petiolaris and H. annuus where a hybrid of the two (H. anomalus) is able to thrive on sand dunes that are otherwise inhospitable to either parent species (Rieseberg et. al., 1995; Rieseberg et. al., 2003). However such shuffling and trading of genes and the generation of harlequin species makes using genes as a method to identify species, or differentiate between natural and escapee populations, quite problematic. To add to this difficulty is the huge plasticity of sunflowers: their ability to modify their shape and form depending on the conditions they find themselves growing in. Undomesticated plants are typically straggly, multiply branched, and have several flowers, seldom larger than a palm-span across. This is in comparison to cultivated plants that have desirable traits for farming: tall and straight (space efficient) with a single large flowerhead, yielding plenty of large seeds. Yet domesticated plants that escape soon return to something resembling their wild relatives and ancestral form. Quite how this plasticity is held in the genes of these plants (and indeed other plants and animals that respond in similar ways to their environments) is of ongoing interest to scientists. Thus, quite what the family history and immigration story of the plants that I saw in Arizona is, I will probably never know, but every year, the shuffle of genes between plants, across populations, between species, across landscapes will continue, and allow bright splashes of yellow to light up the fields and roadsides. Blackman et. al., 2013; Sunflower domestication alleles support single domestication center in eastern North America; Proceedings of the National Academy of Sciences of the USA, 108 (34) Kane & Rieseberg, 2008. Genetics and evolution of weedy Helianthus annuus populations: adaptation of an agricultural weed. Molecular Ecology, 17 (1) Rieseberg et. al., 1995; Hybrid speciation accompanied by genomic reorganization in wild sunflowers; Nature, 375 Rieseberg et. al., 2003; Major Ecological Transitions in Wild Sunflowers Facilitated by Hybridization; Science, 301 (5637)
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