Today we have an entry written by Taisha prior to her departure. Taisha wrote:
Ipomoea purpurea, or the common morning glory, is photographed here by Hugh Nourse (aka Hugh and Carol Nourse@Flickr). This photo was taken last month along the Blue Ridge Parkway in North Carolina. Many thanks for sharing, Hugh!
The bindweed or morning-glory family consists of over 1600 species, including Ipomoea purpurea. This species is an annual vine native to Mexico and Central America, but it has established elsewhere (e.g., in much of the USA and parts of Canada). This species is cultivated worldwide as an ornamental due to its large and showy trumpet-shaped flowers. The Mexican populations are monomorphic for flower colour (having only purple flowers), whereas the introduced populations in the USA are highly polymorphic for flower colour, ranging from white to pink to red and purple. Common morning glory possesses a number of weedy traits such as rapid and aggressive growth, high seed output, and extended seed dormancy in soil seed banks. Indeed, this opportunistic colonizer of disturbed sites is now regarded as a noxious weed in the southeastern United States.
Little is known about the timing and geographic origin of US populations. One hypothesis is that this species was introduced into the United States with maize cultivation ~4000 years ago and subsequently dispersed (called the “Maize migration hypothesis”). Another hypothesis is that it was introduced through Europe after European contact with the New World (termed the “European migration hypothesis”). In this scenario, Spanish explorers are suspected of collecting seeds of Ipomoea purpurea in its native range, sending them to Europe to plant in monastery gardens, and then later being introduced to colonial North America (north of Mexico) around 1700.
In a study by Fang et al., the researchers examined data from 11 loci and 30 Ipomoea purpurea accessions from the native range of the species in Central and Southern Mexico, and 8 accessions from the southeastern USA in an attempt to infer the precise Mexican origins of the southeastern USA populations.
Fang et al. assert that Ipomoea purpurea made its way to southeastern USA via Europe. They support the European migration hypothesis due to the low genetic diversity of the southeastern USA population compared to Mexican populations. This suggests a strong founder effect consistent with multiple founder events (e.g. Mexico to Europe and Europe to southeast USA). This is paired with a severe population bottleneck, which contrasts with the maize diversity in the USA. If common morning glory did migrate with maize after all, one would expect a larger diversity, given the shared demographic history of morning glory and maize. Fang et al. attribute the higher levels of flower colour polymorphisms in the southeastern USA compared to native Mexican populations to the desire for diverse flower colour, consistent with the introduction of this species from Europe to the USA for its horticultural appeal.
As for the origins of the southeastern USA populations, the researchers suggest the western Mexican populations (rather than the southern or eastern). They propose this origin based on their genetic assignment analysis, haplotype composition, and the degree of shared polymorphism. They note that the eastern Mexican populations likely did contribute genetically, but the western populations contribute in large part for three reasons. First, the western Mexican population has the highest diversity and thus a larger effective population size. Secondly, based on the European migration hypothesis, the early trade routes from Mexico to Spain were from the Valley of Mexico eastwards through Xalapa and onto the port of Veracruz. Lastly, they point out that the domestication of maize was ~9000 years ago in the Balsas River Valley, which is ~300km southwest of the Western population. If this were the case, then one would assume the colour variants of Ipomoea purpurea would have spread through Mexico and Central America along with maize culture, rather than the monomorphic purple form.
The researchers end by noting that although it is likely that Ipomoea purpurea first migrated to Europe before being introduced to the southeastern USA, there was substantial trade in the Aztec and earlier eras between the Valley of Mexico and regions to the east and south of Mexico, allowing for seed to be traded before the arrival of Europeans. Also, they mention that the accessions from the Valley of Mexico that show high similarity to the southeastern USA populations could have accompanied modern travelers or been reintroduced back to Mexico to be planted in gardens with likely escapes (see: Fang, Z. et al. (2013). Tracing the geographic origins of weedy Ipomoea purpurea in the Southeastern United States. Journal of Heredity. doi: 10.1093/jhred/est046).