Mate recognition in early lineages of angiosperms:

Most aspects of mate recognition systems cannot be studied in the fossil record. However, there are conspicuous differences between extant gymnosperm and angiosperm lineages with regards to mate recognition. Extant gymnosperms generally do not have pre-fertilization mate recognition, whereas angiosperms generally have varying degrees of both pre- and post-fertilization mate recognition. What kinds of developmental transitions accompanied the origin of pre-fertilization mate recognition in angiosperms? I am addressing this question by studying pre-fertilization mate recognition processes in newly-defined early lineages of angiosperms, focusing initially on self vs. outcross recognition. Because mating systems in plants are mediated by development, not behavior (as in animals), study of mating system evolution is inherently a study of the evolution of development. My lab is undertaking a variety of studies on early lineages of angiosperms, starting with the question of the nature of self versus outcross ontogenies during the progamic phase, the life history period between pollination and fertilization. Mating systems can be strongly influenced by two types of post-pollination processes: competition among male gametophytes and/or differential attrition of male gametophytes. I am currently focusing on Austrobaileya scandens, a vine endemic to the wet tropics region of northeastern Queensland, Australia. Graduate students, Mackenzie Taylor and Nick Buckley working on their dissertations within the Nymphaeales and Austrobaileyales which are two ancient angiosperm lineages that have bisexual flowers, and hence will shed light on the early evolution of post-pollination inbreeding avoidance. Undergraduates are working on several eumagnoliids.

This project is currently funded by NSF.

The origin of pollen tube growth innovations in flowering plants:

An important outcome of the mate recognition project is that it identified pollen tube growth rate innovations as an early player and perhaps a causal mechanism that enabled a host of other reproductive novelties to evolve. Early-divergent angiosperms, such as Amborella, Trithuria, Austrobaileya, Hedyosmum and others, all have similar structural features of the growing pollen tube, the most conspicuous of which are callose tube walls, callose plugs and very thin pectic tips. Gymnosperm pollen tubes are quite different in structure and cannot grow fast or far. Early angiosperm pollen tubes are distinguished by faster growth rates than any gymnosperm pollen tube, and the origin of faster growth rates has been followed by the repeated evolution of long-distance growth and much faster growth rates in derived lineages. The structural characteristics of pollen tubes preceded and seem to have facilitated the origin of carpel closure itself, as well as stylar elongation, ovary elongation and extreme life history acceleration – all traits that can be said to have contributed to angiosperm diversification.

In this project we are studying sources of ontogenetic and life history variation in pollen tube growth rates across a number of early-divergent angiosperms. A number of ecological conditions impose strong selection for the evolution of rapid life histories, and the speeding of such life histories in angiosperms invariably involved speeding of pollen tube growth rates. How do early angiosperm lineages evolve faster rates, and what are the limitations or pathways that prevent or enable such rates to evolve? Gymnosperm growth rates have been fairly resistant to evolutionary change. In concert with our comparative developmental analyses, a post-doc on this project is using genomic resources from Pinus, Amborella and Nuphar, along with transcripts from their growing pollen tubes, to find some of the structural genes involved in producing the novel growth features of angiosperm pollen tubes. The gene families we are starting with are: CalS, involved in synthesis of callose in pollen tube wall, and PME, involved in de-esterification of pectins just behind the growing tip. We have hypothesized that gene duplication played a significant role in the origin of angiosperm pollen tube growth, at the structural and/or the regulatory gene level, and hence in the adaptive radiation of flowering plants.

Phylogeography of Austrobaileya scandens:

In collaboration with Andrew Ford (CSIRO, Atherton, Australia), I am tracing the patterns of genetic diversity in this relict species. Austrobaileya is the sole species within the family Austrobaileyaceae, however its range is today strongly partitioned into two separated distributions. Most populations occur on the eastern side of the small mountain range just southwest of Cairns city, one of the wettest places on earth. The only other populations known, occur in an isolated area of highlands from Mount Lewis and to the northwest on the Carbine plateau. These two areas are separated by the Black Mountain Divide. Formerly some individuals from the Carbine area (Mt. Spurgeon) were described as a separate species, Austrobaileya maculata. To date, we have used nuclear microsatellite and chloroplast PCR-RFLP markers to examine historical patterns of genetic diversity and inbreeding.

Phylogeography of Betula neoalaskana (=B. resinifera):

I have a long-standing interest in the oaks and birches. These open-pollinated trees often have huge geographic ranges, are highly outcrossing and frequently hybridize. Understanding how diversity originates and is maintained in these organisms is quite interesting since most of their life history characters mediate against population divergence (and speciation) even while promoting within-population genetic diversity. A long standing challenge for biologists has been where Alaskan tree species survived the ice ages. It was long thought that trees could not have survived glacial periods as far north as Alaska/Beringia and hence that most tree species either went through periodic local extinctions or range contractions to warmer areas south of the ice sheets. Genetic data from herbaceous plants and some animals seems to indicate that  many organisms survived in Alaska/Beringia during glacial periods. Did the Alaska paper birch and other tree species also survive there? I have been using chloroplast DNA (PCR-RFLPs and microsatellites) as well as nuclear microsatellite data to test alternative refugium hypotheses (Beringia-only, South of the Ice-only, or both). This study is in collaboration with University of Alaska researchers Tom Clausen and John Bryant who have studied geographic patterns of browsing-resistance chemistry that may have evolved in a very few birch generations. See Bryant et al. 2009 on publications page.

Publicity

Mayfield, J. 2009. Flora in Excelsis. Quest (Research magazine of University of Tennessee) 1(2): 8-11.

Bosveld, J. 2009. Speedy sperm explains flower power (Top 100 science stories of 2008, number 87). Discover magazine January issue, page 69. See article

Smith, W. 2009. Darwin would be proud. Bearden/Cedar Bluff Shopper-News Now January 19, 2009, page A-5. See article

Carlson, E. 2009. Professor's research receives honor. The Daily Beacon (University of Tennessee) 1/9/09. See article

Williams, R. D. 2008. Discover magazine honors UT professor. Knoxville News Sentinel 12/26/08. See article

Minas Gerais TV Rede Integração. 2008. Espécie rara de árvore de 125 milhões de anos é encontrada no Cerrado. Interview in Portuguese from local TV in Uberlandia, Brasil. See article

University of Tennessee internal publicity

    Mayfield Dec. 15, 2008    

    Mayfield July 29, 2008

Manjimup Times (Western Australia) article on Mackenzie Taylor's fieldwork, December, 2008. See article