A MOLECULAR REEXAMINATION OF INTROGRESSION BETWEEN HELIANTHUS ANNUUS AND H. BOLANDERI (COMPOSITAE)

Abstract

Abstract. -Heiser (1949) hypothesized that a weedy race of Helianthus bolanderi had originated by the introgression of genes from H. annuus into a serpentine race of H. bolanderi. Although Heiser's investigation of these species is frequently cited as one ofthe best examples ofintrogression in plants, definitive evidence of gene exchange is lacking (Heiser, 1973). To determine whether the weedy race of H. bolanderi actually originated via introgression, we analyzed allozyme, chloroplast-DNA (cpDNA), and nuclear-ribosomal-DNA (rDNA) variation.Evidence from enzyme electrophoresis did not support the proposed introgressive origin ofweedy H. bolanderi. We detected a total of 37 low-frequency alleles distinguishing the serpentine race of H. bolanderi from H. annuus. Weedy H. bolanderi possessed only four of the 37 marker alleles. Further analysis demonstrated that serpentine H. bolanderi combined seven of the 35 alleles distinguishing H. annuus from weedy H. bolanderi, indicating that serpentine H. bolanderi shares three more marker alleles with H. annuus than does weedy H. bolanderi. These results are similar to expectations for race divergence from a single common ancestor and suggest that, ifintrogression occurred, the majority of marker alleles were rapidly lost following the initial hybridization event.Even more compelling evidence opposing Heiser's (1949) hypothesis, however, was from restriction-fragment analysis of cpDNA and nuclear rDNA. We detected a total of 17 cpDNA and five rDNA restriction-site mutations among the 19 populations examined. No parallel or back mutations were observed in phylogenetic trees constructed using eithercpDNA or rDNA mutations, and both phylogenies were completely congruent regarding the alignment of all three taxa. In addition, the weedy race of H. bolanderi possessed a unique cpDNA, which was outside the range of variation observed among populations of either ofthe presumed parental species. Mean sequence divergences between the cpDNAs of weedy H. bolanderi and those of serpentine H. bolanderi and H. annuus were 0.30% and 0.35%, respectively. These estimates are comparable to sequencedivergence values observed between closely related species in other plant groups. Given the lack of parallel or convergent mutations in the cpDNA and rDNA phylogenetic trees, the complete congruence of these trees with flavonoid-and allozyme-variation patterns, and the presence of a unique and divergent chloroplast genome in the weedy race of H. bolanderi, we suggest that the weedy race of H. bolanderi was not derived recently through introgression, as hypothesized, but is relatively ancient in origin.

Storage effects on genomic DNA in rolled and mature coca leaves

“…This is particularly true for taxa of Erythroxylum where the obstacles demarcated by Plowman and Rivier (1) still exist today. Several methodologies are available that address the preservation of plant material for molecular systematic studies for plants indigenous to tropical regions (2)(3)(4)(5)(6) as well as descriptions of their performance (5,7,8). The most efficient method for preserving plant material is with the use of liquid nitrogen, but this imposes inherent problems in tropical environments (2).…”

Re-evaluating Homoploid Reticulate Evolution in Helianthus Sunflowers

“…Early studies documented hybrid swarms between different sunflower species ( Heiser 1947 ; Heiser 1949a ; Heiser 1951 ), demonstrating that chromosomal rearrangements contributed to hybrid sterility ( Chandler et al 1986 ; Lai et al 2005 ), and speculated on possible ecotypes and species that might be the products of hybridization ( Heiser 1949b ; Heiser 1958 ; Rieseberg 1991 ). Later molecular phylogenetic work and early low-resolution genomic studies supported some of these hypotheses ( Rieseberg, Beckstrom-Sternberg, et al 1990 ; Yatabe et al 2007 ), but not others ( Rieseberg et al 1988 ), and also suggested new hypotheses, including the putative origins of three homoploid hybrid species ( Rieseberg 1991 ). Here, we have taken advantage of new high-resolution genomic data and computational methods to further explore the role of hybridization in sunflower evolution.…”