Inflorescence of Ceiba pentandra

Flower of Delonix regia

Pollen from Delonix regia stained to estimate viability; viable grains are dark blue, nonviable grain is pale blue, air bubble is in lower left.

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Douglas G. Scofield
Postdoctoral Research Associate
dgscofie(at)indiana.edu
(812) 856-0115 (812) 219-5373

Ph.D., Biology, University of Miami, Coral Gables, Florida, 2004 (Award of Academic Merit).
        Thesis title: Reproductive consequences of mutation in long-lived plants.
B.S., Botany, Florida Atlantic University, Boca Raton, Florida, 1997 (summa cum laude).
B.S., Computer Science, Michigan State University, East Lansing, Michigan, 1988.

Figure: Median intron size throughout the 5' UTR and CDS of four model organisms. We have proposed an evolutionary model that explains the increased size of introns in the 5' UTR and the sharp drop in intron size at the 5' UTR-CDS boundary via differing strengths of selection against intron splice site shifts within the 5' UTR and CDS, arising from sequence constraints and the presence of potentially deleterious premature start codons (uAUGs) upstream of the true Start codon (Mol. Biol. Evol. 23:2392-2404).

My doctoral dissertation (supported by NSF Doctoral Dissertation Improvement Grant DEB-0309253) examined the reproductive consequences of mitotic mutation in large-statured plants. A number of lines of evidence indicate that long-lived plants have higher per-generation mutation rates than short-lived plants. This is believed to be a consequence of an increased contribution of mitotic mutation, due to plants' lack of a segregated germ line, such that gametes are produced from cell lineages that have undergone hundreds to hundreds of thousands of potentially imperfect mitoses prior to meiosis. More mitoses would give rise to more mitotic mutation, leading to the expectation of higher per-generation mutation rates (U) in long-lived, large-statured plants such as trees. Although this expectation is not itself novel, the consideration of its consequences for reproductive responses of trees has received little attention.

Figure: Sources of de novo mutation load due to mitotic mutation in a growing plant. Flower locations are indicated in red, and the slope of the line is equal to the mitotic mutation rate us.

I used a combination of fieldwork, labwork, theoretical modelling and meta-analysis to develop a general model explaining stature-based differences in evolutionary constraints on plant mating systems. The Φ model of plant mating system evolution assumes that the per-generation mutation rate is a positive function of Φ, the number of mitoses that occur in a plant's lifetime from zygote to gamete production. The association between the Greek letter Φ and mitosis is reflected in its resemblance to a dividing cell. The Φ model predicts that while small-statured (`low-Φ') plants such as herbs are free to have a mating system that includes selfing, large-statured (`high-Φ') plants such as trees have a per-generation mutation rate that is too high to allow for selfed progeny to reach reproductive maturity under nearly all natural conditions.

Figure: Population genetic data supporting the Φ model of plant mating system evolution. Data are adult inbreeding coefficient (F) vs. progeny selfing rate (S) measured in the same population of a variety of species of (a) small-statured, `low-Φ plants, and (b) large-statured, `high-Φ' plants; all data from published studies. Lines represent model fits as determined by several genetic models for the evolution of inbreeding depression. For further details see Proc. R. Soc. Lond. B 273:275-282. The dataset used to generate the figure is available as a Supplemental Appendix.

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Because Φ is a critical component of the model, I'm developing methods for estimating Φ itself. I've started by estimating Φ in D. regia using mature medial pith cells in twigs; for more details see the forthcoming Am. J. Bot. 93:1740-1747.

In addition to the development of the Φ model, I developed two techniques for estimating rates of mitotic mutation in trees: the sexual progeny assay (first proposed by Klekowski in his 1988 book) that relies upon fitness differences between selfed progeny created from gametes belonging to different cell lineages within the same tree; and the pollen fitness assay, that tests for within-branch declines in pollen fitness. I applied the sexual progeny assay to Ceiba pentandra in Chamela, Mexico, and the pollen fitness assay to Delonix regia in Miami, Florida. Although final analyses are still underway, preliminary results indicate that plants have a per-mitosis mutation rate that is at least one to two orders of magnitude lower than in animals.

Figure: Sexual progeny assay. Within-flower selfed progeny (A and B) are created via pollination of a flower with its own pollen, while between-flower selfed progeny are created via pollination of a flower with pollen from a different flower borne either born on the same branch (B×A) or on a different primary branch (A×A). Within-flower selfed progeny are expected to have lower fitness than between branch selfed progeny due to a higher number of de novo mitotic mutations shared by the ovules and pollen grains, and there are a number of ways to combine fitness observations to derive estimates of mutation parameters.