User:Ziqixie/Natural hybridization in Louisiana irises

As plants generally have lower reproductive barriers between closely related species, lots of horticultural and agricultural hybrid plant lineages have been produced artificially. In nature, many plant hybrid zones have also been recorded and have become the basis of the development of major hybrid zone theories.

Hybridization of Louisiana irises of the Mississippi Delta have been extensively studied since the 1930s. In illustration of his introgressive hybridization theory, Edgar Anderson used the hybrid zone between Iris fulva and Iris hexagona var. giganti-caeulea (HGC) as an example, based on a previous study done by Herbert Riley (1938). I. fulva was a wide-range species that can be found in wet clay soil (bayou system) from Ohio River valleys to the lower Delta of the Mississippi, where the smaller-range HGC grew in alkaline marshes. The two species came into contact in the lower Mississippi Delta between New Orleans and the sea, which is characterized by branching rivers with constantly changing courses and lots of agricultural development by small farms in between the rivers and bayous. (Viosca 1935) I. fulva is characterized by its small, red flowers, while HGC has large flowers with white to blue colors and a patch of bright yellow. In the early 1900s, irises of various colors grown in the natural areas of this region attracted the attention of lots of local gardeners. Later, John Small illustrated and described these diverse irises as newly discovered species (Small 1927, Small and Alexander 1931). Subsequent investigations by other botanists (Visosca 1935, Foster 1937) raised questions as to how many of these iris variations could be defined as species. Riley’s study (1938) took place in an abandoned deltaic stream with two levees, on top of one of which was small farms and public roads. One of the bayous of the river cut across these levees and formed a wide marsh, where HGC populations inhabit. Along the edge of the stream are sporadically growing I. fulva for several miles. The hybrids were mainly found in two groups (H-1 and H-2), where individuals bearing intermediate characteristics in terms of flower size, color and assortment were found. Riley recorded a series of morphological traits such as color of sepal blade, sepal length and petal shape as well as pollen fertility of the two parental populations and the two putative hybrid populations (1938). He then performed a hybrid index analysis developed by Anderson (1936) by arbitrarily assigning values to the morphological characters, with one of the parental species individuals associated with a higher value and the other associated with a lower value. According to the results, a number of hybrids were identified from both of the two putative hybrid habitats. Noting the limited range of the H-1 habitat, Anderson used the Louisiana iris hybrid zones as a case of “hybridization of the habitat” (1948, 1949), where either an intermediate habitat has to exist for the F1 hybrids to establish, or some F1 hybrids have to be similar enough to one of the parents in order to sustain in the parental habitat.

While the morphological biogeography of hybrid zones between I. fulva and HGC was used by Anderson (1949) as an illustration for introgressive hybridization, as a lot of other putative hybrid populations and habitats demonstrated similar patterns. A later cytological study denied the presence of genetic material exchange, and hence introgression, between the two species (Randolph et al. 1967). As the original study site of Riley (1938) has vanished due to increased human settlement, other putative hybrid populations in Southern Louisiana were chosen for study. The new study site contained three species, I. fulva, I. brevicaulis, and I. giganticaerulea (HGC) [add description of each of their habitats]. Natural hybrids of I. fulva and I. brevicaulis, and of I. fulva and I. giganticaerulea were identified based on the intermediate flower color and size they demonstrated. The researchers also sampled from allopatric populations of all three species in Southwestern Louisiana. [more to write about the result analysis]

[Arnold et al. (1990) showed through genetic marker that introgressive hybridization did take place between the populations studied by Randolph et al. 1967]

[Cruzan and Arnold (1993): non-random association between hybrid genotype and parental habitat]

The continued research done on Louisiana iris hybrid zones across the century illustrates how biologist’s understandings of natural hybridization events have developed alongside advancements in molecular technologies.