Spring 2016 Student-Invited Speaker Is Maryn Carlson
Posted: March 29, 2016
On March 28, 2016, M.S. candidate Maryn Carlson of Cornell University visited the Department of Plant Pathology and Environmental Microbiology (PPEM) at Penn State as part of the student seminar exchange program between the two schools. Maryn met with students and faculty throughout the day before sharing her research seminar focused on identifying genetic changes in a controlled population of Phytopthora capsici, responsible for phytophthora blight in cucurbit, solanum, and bean. This pathogen, capable of causing extreme crop loss, has become increasingly common and difficult to manage in recent years. She and her fellow colleagues believe their research will improve our understanding of the pathogen’s long-term development in the field and may lead to better management strategies.
(L-R) Garrett Morrison, Edelio Bazán, Maryn Carlson, Freddy Magdama, Laura del Sol Bautista Jalón, Genna Tesdall, Jennie Mazzone, and Rob Harvey
Maryn Carlson is a graduate student at Cornell University working with Dr. Christine Smart on the genetics of Phytophthora capsici, a devastating pathogen that causes Phytophthora blight on many vegetable crops. Her research focuses on the genetics of P. capsici on both individual and population scales, using genomic, single-nucleotide polymorphism data to study both a controlled research field population and a novel infestation of P. capsici in a grower’s field. Using these data, she also investigates the genetic basis of mating type in the heterothallic P. capsici.
The common co-occurrence of both A1 and A2 mating types of Phytophthora capsici results in production of oospores, and consequently persistent, overwintering populations in many locations. To understand the dynamics of these often isolated, sexual populations, a restricted access research farm in Geneva, NY, with no prior history of P. capsici, was inoculated in 2008 with two isolates of opposite mating type. Approximately fifty isolates were sampled each year, from 2009-13 from a variety of plant species. To provide a controlled reference population for the field study, F1 single-oospore progeny were isolated from an in vitro cross between the same founding parents. Isolates were analyzed using genotyping-by-sequencing (GBS), which simultaneously identifies and scores single nucleotide polymorphism (SNP) markers, resulting in approximately 25,000 genome-wide SNP markers. Indicators of inbreeding, multi-locus heterozygosity and the inbreeding coefficient, show that over time the population underwent a generational shift; transitioning from putative F1 in 2009-10, mixed generational in 2011, and ultimately all inbred in 2012-13. In addition, a lower frequency of selfed isolates in the field relative to the in vitro cross may suggest a fitness cost to selfing manifest in the field but not the lab. Capitalizing on the segregation of mating type in this population, we performed a genome-wide association study to identify loci associated with mating type. Our results support a hypothesis for four segregating haplotypes in the mating type region, potentially defined by structural variation and suppressed recombination. An analogous GBS approach was applied to seventy isolates sampled from a P. capsici population in a newly infested grower's field of cucurbits. Our assessment of population structure and genetic distance in this population show that there were several founding isolates, with asexual reproduction resulting in the proliferation of a subset of these founders. Analysis of inbreeding and mating type inheritance in this unique, closed, bi-parental field population in conjunction with the investigation of an on-farm population offers novel insights into the genetics of P. capsici on population and individual scales.