Paul G. Wolf - Utah State University, Dept. Biology - Research web site

Hybrid zones in Ipomopsis

One of my research projects is an extension of my thesis work on Ipomopsis (Polemoniaceae).  I am interested in the reproductive barriers that are evolving in this species complex, especially those mechanisms associated with pollinators.   I am examining chloroplast DNA variation across the range of I. aggregata (red-flowered, hummingbird-pollinated) and I. tenuituba (white-flowered, moth-pollinated).  These two species come into contact at about 30 sites throughout the western U.S.  In some of these areas they hybridize and in other they do not.  I have detected 23 chloroplast DNA mutations from samples at 12 sites.  All mutations unite geographic areas rather than populations of a single species.  Unfortunately I do not (yet) have useful data from nuclear markers so it is difficult to infer whether the patterns result from introgression or if the floral differences between the species have evolved independently in each hybrid zone.  Currently, I am focusing on contact zones between I. aggregata and I. arizonica in southwestern U.S.  Both these species are red-flowered and appear to be bird-pollinated, and I have observed birds visiting both species at the contact zones.  However, hybrids have not been reported from natural populations.  Flowers of I. arizonica are short (about 10 - 15 mm), whereas those of I. aggregata are longer (18-32mm) Studies with captive hummingbirds (in collaboration with Nick Waser and Diane Campbell) suggest that the birds can transfer pollen from I. arizonica to I. aggregata but not the other way.  With help from Trent Toler and Jenny Archibald,  we examined pollen competition between the two species.  Pollen from I. arizonica was unable to fertilize I. aggregata unless the style was cut short.  However, pollen from I. aggregata could fertilize I. arizonica, even when competing with pollen from I. arizonica.  Thus, the reproductive isolation between the two species appears to be asymmetric with pollinator effectiveness acting in one direction and limitations of pollen (perhaps through pollen tube growth) acting in the other direction.  Future plans include a geographic study of nuclear and chloroplast DNA variation to address the question of whether introgression has occurred and if so in which direction.  I hypothesize that pollen limitation will be more effective than pollinator effectiveness and so introgression from I. aggregata to I. arizonica may be possible.

Published Ipomopsis papers:

Wolf, P. G.,  D. R. Campbell, N. M. Waser, S. D. Sipes, T. R. Toler, and J. K. Archibald. 2001.  Tests of pre-and post-pollination barriers to hybridization between sympatric species of Ipomopsis (Polemoniaceae). American Journal of Botany 88: 213-219. View Abstract

Campbell, D. R., N. M. Waser, P. G. Wolf.  1998.  Pollen transfer by natural hybrids and parental species in an Ipomopsis hybrid zone.  Evolution 52: 1602-1611.  

Wolf, P. G., R. A. Murray, and S. D. Sipes.  1997. Species-independent, geographic structuring of chloroplast DNA haplotypes in a montane herb, Ipomopsis (Polemoniaceae).  Molecular Ecology 6: 283-291.  

Wolf, P. G. and D. R. Campbell. 1995. Hierarchical analysis of allozymic and morphometric variation in a montane herb,Ipomopsis aggregata (Polemoniaceae). Journal of Heredity 86: 386-394.

Wolf, P. G., P. S. Soltis and D. E. Soltis. 1993. Phylogenetic significance of chloroplast DNA restriction site variation in the Ipomopsis aggregata complex and related species (Polemoniaceae).  Systematic Botany 18: 652-662

Wolf, P. G. and P. S. Soltis. 1992.  Estimates of gene flow among populations, geographic races, and species in the Ipomopsis aggregata complex (Polemoniaceae). Genetics 130: 639-647.  

Wolf, P. G., P. S. Soltis and D. E. Soltis. 1991. Genetic relationships and patterns of allozyme divergence in the Ipomopsis aggregata complex and related taxa (Polemoniaceae). American Journal of Botany 78: 515-526.

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