Ph.D., Univ. of Colorado 1987
Univ. of New Mexico 1988-90
Office: Lapham S585
Research in my lab group focuses on evolutionary processes occurring in flowering plant populations. My students and I are particularly interested in understanding how plant mating systems evolve and how patterns of pollinator visitation influence male and female reproductive success. In collaboration with Dr. Randy Mitchell (Univ. of Akron) we have studied how ecological factors, such as population density and the presence of competitors for pollination, and genetic factors, such as floral morphology and floral display size, influence selfing rates and patterns of paternity in monkeyflower (Mimulus ringens). At our study site in SE Wisconsin this wetland perennial is pollinated by five sympatric species of bumble bees (Bombus). Through the use of genetic markers to unambiguously determine paternity, we have developed an unparalleled data set documenting fine-scale variation in mating patterns. For example, we have discovered that adjacent flowers open on the same day differ strikingly in selfing rates and number of outcross pollen donors. Nearly all fruits are multiply sired, averaging 4.92 outcross donors per fruit. With support from the National Science Foundation we are exploring the following questions:
1) What mechanisms are responsible for the high levels of multiple paternity observed in Mimulus ringens fruits?
In Mimulus ringens multiple paternity results from two mechanisms. When a pollinator probes a flower, it deposits a mixture of pollen from 1-3 sires. Within 20 minutes of the first probe, a second pollinator often visits the same flower and deposits pollen from 1-3 additional pollen donors. See Karron et al. 2006 PDF.
2) Why do selfing rates of flowers from the same daily floral display vary so dramatically?
Selfing rates of individual flowers are influenced by the amounts of within-flower (autogamous) and among-flower (geitonogamous) self-pollen deposited on stigmas. In a pollinator visitation sequence the first flower probed is predominantly outcrossed, with just 20% of the ovules fertilized by autogamy. Since pollen carryover is very limited, flowers visited sequentially receive increasing proportions of geitonogamous self pollen. Geitonogamous selfing rates are therefore highest on large floral displays. See Karron et al. 2004 PDF See also Karron et al. 2009 PDF.
3) How does floral display size influence patterns of pollinator visitation?
Bumble bees strongly prefer large Mimulus ringens floral displays, and probe more flowers in sequence on large displays than on small displays. See Mitchell et al. 2004 PDF
4) How does floral display size influence male and female fitness?
Analysis of total fertility through male and female function confirms that plants with larger displays mother and sire more seeds. However, siring success per flower declines sharply with increasing display size, while female success per flower does not vary with display. Total fitness per flower (accounting for greater expression of inbreeding depression with increased selfing rate) also declines strongly with floral display.
5) What is the extent of pollen carryover in Mimulus ringens?
Dr. Karsten Holmquist demonstrated that pollen carryover is very limited in Mimulus ringens, and most genes are dispersed to the first three recipient flowers in the visitation sequence.
Dr. Holmquist received his PhD in 2005 and is now a post-doctoral researcher at Univ. of Wisconsin-Madison.
6) How do competitors for pollination influence reproductive success and the mating system?
Dr. John Bell demonstrated that the presence of Lobelia siphilitica, a competitor for pollination, leads to a 37% reduction in Mimulus seed set. In addition, Mimulus had a significantly lower rate of outcrossing when grown in competition with Lobelia. This is the first study to demonstrate that competition for pollination directly influences outcrossing rates. See Bell et al. 2005 PDF
Dr. Bell received his PhD in 2004 and is now Vice President of Applied Ecological Services, one of the largest environmental restoration companies in North America.
Dr. Rebecca Flanagan has been studying how multiple species interact when competing for pollinators. By establishing arrays with different combinations of Mimulus ringens, Lobelia siphilitica, and invasive Lythrum salicaria, she has explored whether the effects of these competitors on Mimulus are additive or synergistic. She has also quantified mechanisms of competition for pollination. Rebecca's work provides the first demonstration that grooming by bees as they forage on a competitor may significantly reduce seed set in a focal species. See Flanagan et al. 2009 PDF
Dr. Flanagan received her PhD in 2009 and is now a post-doctoral researcher at Indiana University.
PROSPECTIVE GRADUATE STUDENTS: My lab will be accepting two graduate students to start in Fall 2010. I give all of my graduate students the freedom to develop independent dissertation research projects.
Flanagan RJ, Mitchell RJ, and Karron JD. 2010. Increased relative abundance of an invasive competitor for pollination, Lythrum salicaria, reduces seed number in Mimulus ringens. OECOLOGIA in press. ABSTRACT
Flanagan RJ, Mitchell RJ, and Karron JD. 2010. Effects of multiple competitors for pollination on bumblebee foraging patterns and Mimulus ringens reproductive success. OIKOS in press. ABSTRACT
KARRON, J. D., K. G. Holmquist, R. J. Flanagan, and R. J. Mitchell. 2009. Pollinator visitation patterns strongly influence among-flower variation in selfing rate. Annals of Botany 103: 1379–1383. PDF COVER PHOTO
Mitchell, R. J., R. J. Flanagan, B. J. Brown, N. M. Waser, and J. D. KARRON. 2009. New frontiers in competition for pollination. Annals of Botany 103: 1403–1413. PDF
Flanagan, R. J., R. J. Mitchell, D. Knutowski, and J. D. KARRON. 2009. Interspecific pollinator movements reduce pollen deposition and seed production in Mimulus ringens (Phrymaceae). American Journal of Botany 96: 809–815. PDF
Mitchell, R. J., R. E. Irwin, R. J. Flanagan, and J. D. KARRON. 2009. Viewpoint: Ecology and evolution of plant-pollinator interactions. Annals of Botany 103: 1355-1363 PDF
KARRON, J. D., R. J. Mitchell, and J. M. Bell. 2006. Multiple pollinator visits to Mimulus ringens (Phrymaceae) flowers increase mate number and seed set within fruits. American Journal of Botany 93: 1306-1312. PDF
Bell, J. M., J. D. KARRON, and R. J. Mitchell. 2005. Interspecific competition for pollination lowers seed production and outcrossing in Mimulus ringens. Ecology 86: 776-785. PDF
Mitchell, R. J., J. D. KARRON, K. G. Holmquist, and J. M. Bell. 2005. Patterns of multiple paternity in fruits of Mimulus ringens (Phrymaceae). American Journal of Botany 92: 885-890. PDF
KARRON, J. D., R. J. Mitchell, K. G. Holmquist, J. M. Bell, and B. Funk. 2004. The influence of floral display size on selfing rates in Mimulus ringens. Heredity 92: 242-248. PDF
Mitchell, R.J., J. D. KARRON, K. G. Holmquist, and J. M. Bell. 2004. The influence of Mimulus ringens floral display size on pollinator visitation patterns. Functional Ecology 18: 116-124. Article PDF Journal Cover Photo
Linhart, Y. B., L. M. Ellwood, J. D. KARRON, and J. L. Gehring. 2002. Genetic differentiation in the dwarf mistletoes Arceuthobium vaginatum and Arceuthobium americanum on their principal and secondary hosts. International Journal of Plant Sciences 164: 61-69.
Reinartz, G. E., J. D. KARRON, R. B. Phillips, and J. L. Weber. 2000. Patterns of microsatellite polymorphism in the range-restricted bonobo (Pan paniscus): considerations for interspecific comparison to chimpanzees (P. troglodytes). Molecular Ecology 9: 315-328 .
KARRON, J. D. 1998. Genetic consequences of different patterns of distribution and abundance. Chapter 10 in The Biology of Rarity. W. E. Kunin and K. J. Gaston, eds. Chapman & Hall, London, pp. 174-189.
KARRON, J. D., R. T. Jackson, N. N. Thumser, and S. L. Schlicht. 1997. Outcrossing rates of individual Mimulus ringens genets are correlated with anther-stigma separation. Heredity 79: 365-370.PDF
Thumser, N. N., J. D. KARRON and M. S. Ficken. 1996. Interspecific variation in the calls of Spheniscus penguins. The Wilson Bulletin 108: 72-79.
KARRON, J. D., R. Tucker, N. N. Thumser and J. A. Reinartz. 1995. Comparison of pollinator flight movements and gene dispersal patterns in Mimulus ringens. Heredity 75: 612-617. PDF
KARRON, J. D., N. N. Thumser, R. Tucker and A. J. Hessenauer. 1995. The influence of population density on outcrossing rates in Mimulus ringens. Heredity 75: 175-180. PDF
KARRON, J. D. and D. L. Marshall. 1993. Effects of environmental variation on fitness of singly and multiply sired progenies of Raphanus sativus (Brassicaceae). American Journal of Botany 80: 1407-1412. PDF
KARRON, J. D. 1991. Patterns of genetic variation and breeding systems in rare plant species. Chapter 6 in Genetics and Conservation of Rare Plants. D. A. Falk and K. E. Holsinger, eds. Oxford University Press, Oxford.
KARRON, J. D. and D. L. Marshall. 1990. Fitness consequences of multiple paternity in wild radish, Raphanus sativus. Evolution 44: 260-268. PDF
KARRON, J. D, D. L. Marshall and D. M. Oliveras. 1990. Numbers of sporophytic self-incompatibility alleles in populations of wild radish. Theoretical and Applied Genetics 79: 457-460. PDF
KARRON, J. D. 1989. Breeding systems and levels of inbreeding depression in geographically restricted and widespread species of Astragalus (Fabaceae). American Journal of Botany 76: 331-340. PDF
KARRON, J. D., Y. B. Linhart, C. A. Chaulk and C. A. Robertson. 1988. Genetic structure of populations of geographically restricted and widespread species of Astragalus (Fabaceae). American Journal of Botany 75: 1114-1119. PDF
KARRON, J. D. 1987. A comparison of levels of genetic polymorphism and self-compatibility in geographically restricted and widespread plant congeners. Evolutionary Ecology 1: 47-58. PDF
KARRON, J. D. 1987. The pollination ecology of co-occurring geographically restricted and widespread species of Astragalus (Fabaceae). Biological Conservation 39: 179-193.
Dissertations and theses by graduate students in my lab:
Flanagan, R. J. (PhD 2009). Exploring the effects of competitors for pollination on the reproductive success of Mimulus ringens.
Holmquist, K. G. (PhD 2005). The effect of floral display and pollinator behavior on pollen-mediated gene dispersal in Mimulus ringens.
Bell, J. M. (PhD 2003). The effect of interspecific competition for pollinator service on seed production and outcrossing rates in Mimulus ringens (Scrophulariaceae).
Artiomow, H. (MS 2002). The influence of population size and density on pollinator service in Agalinis skinneriana (Orobanchaceae).
Berger, R. (MS 1998). Conserving a rare, self-incompatible plant: seed production and clonal structure of Aster furcatus populations.
Reinartz, G. E. (PhD 1997). Patterns of genetic diversity in the bonobo (Pan paniscus).
Thumser, N. N. (PhD 1993). Phylogenetic relationships among Spheniscus penguins based on the analysis of vocal and allozyme data.