Abstract:
Patterns of genetic variation do not arise in a vacuum but are instead shaped by the interplay between evolutionary forces and ecological constraints. Here, I use a phylogeographic approach to examine the role that ecology played in lineage divergence in the Desmognathus quadramaculatus species complex (Family: Plethodontidae), which consists of three nominal species: D. quadramaculatus, D. marmoratus, and D. folkertsi. Previous phylogenetic studies have shown that individuals from these species do not form clades based on phenotype. My approach to reconciling phylogenetic discordance was two-fold, using (1) genome-wide markers to provide insight into the relationships among lineages and (2) geographic and climate data to provide context for patterns of genetic diversity. First, I obtained genome-wide nuclear markers using double digest restriction-site associated DNA sequencing (ddRAD) to examine whether two morphologically divergent species, D. marmoratus and D. quadramaculatus, represent independently evolving lineages. Phylogenetic, population structure, and model testing analyses all confirmed that D. marmoratus and D. quadramaculatus do not group based on phenotype. Instead, I found that there were two cryptic genetic lineages (Nantahala and Pisgah) that each contained both phenotypes. Additionally, ecological niche modeling showed that the two genetic lineages primarily occupy geographic areas with significantly different climates, suggesting that climate may have played a role in divergence. Next, I assembled loci from publicly available sequencing data using a draft transcriptome of Desmognathus fuscus as a reference to assess the three nominal species in the quadramaculatus species complex across their entire range. I used phylogenetic and population structure analyses, alongside haplowebs and conspecificity matrices, to determine if the loci supported the hypothesis that the phenotypes represent multiple independently evolving lineages within the broader genetic clades found in the previous chapter. I found that the loci were not informative enough to determine whether the phenotypes had a genetic basis in Pisgah, but did support genetic divergence between phenotypes in Nantahala. Finally, I used ecological niche models (ENMs) and resistance modeling to place the genetic results and phenotypic diversity within the context of time and space. I found that though the quadramaculatus and marmoratus phenotypes were nearly indistinguishable in niche space in the present day, they were projected to occupy different geographic areas in the past and future. The southern portion of the study area had areas of high habitat suitability from the Last Glacial Maximum (~22 kya) to the present, which aligns with the higher genetic divergence between groups in Nantahala. Anthropogenic land use changes reduced habitat availability but likely did not drive genetic divergence in the past, and may be of more consequence to genetic diversity than climate change over the next 50 years.  Like many taxa that underwent adaptive radiations, the evolutionary history of Desmognathus has been obfuscated by high rates of within-species phenotypic diversity and shared morphology between distantly related lineages. My findings emphasize the importance of interrogating complex patterns of genetic variation within the context of the dynamic, heterogeneous landscapes in which they arise.

Dr. Francois Spitz | Spitz Lab

Bio:
PhD from Université Paris 6 (France)
Group Leader at the European Molecular Biology Laboratory (2006-2015) (Heidelberg, Germany)
Head of Research Unit at the Institut Pasteur (2015-2019) (Paris, France)
Professor, The University of Chicago (2019-.)

Abstract:
The mechanisms that regulate the efficiency and specificity of interactions between distant genes and cis-regulatory elements such as enhancers play a central role in shaping the specific regulatory programs that control cell fate and identity. In particular, the (epi)genetic elements that organize the 3D folding of the genome in specific loops and domains have emerged as key determinants of this process. I will discuss our current views on how 3D genome architecture is organized, how it influences gene regulatory interactions and illustrate how alterations of the mechanisms and elements that organize genomes in 3D could contribute to genomic disorders and genome evolution.

Dr. Blake Meyers | Meyers Lab

BIO: 
Blake Meyers is a Member & Principal Investigator at the Donald Danforth Plant Science Center in St.
Louis, and he is a Professor in the Division of Plant Science and Technology at the University of
Missouri - Columbia. He formerly held the Edward F. and Elizabeth Goodman Rosenberg
professorship at the University of Delaware, where his research group was from 2002 to 2015. He
was elected as a Fellow of the American Association for the Advancement of Science (AAAS) in 2012,
and a Fellow of the American Society of Plant Biologists (ASPB) in 2017, the same year he was
awarded the Charles Albert Shull Award by the ASPB for outstanding investigations in the field of
plant biology. He was elected to the US National Academy of Sciences in 2022. After serving on the
editorial board since 2008, Blake became the Editor-in-Chief of The Plant Cell in January 2020. Work
in his lab addresses the biological functions, biogenesis, genomic impact, and evolution of small
RNAs in diverse plant species, using combination of genomic and molecular genetics approaches,
with a focus on phased, secondary siRNAs (“phasiRNAs”).

He received his undergraduate degree in biology from the University of Chicago in 1992, and
working with Prof. Richard Michelmore at UC Davis, was awarded M.S. and Ph.D. degrees in genetics in 1995 and 1998, respectively. After that, he did a postdoc with Prof. Michele Morgante at DuPont Crop Genetics, working on maize genomics for 2 years before returning to Prof. Michelmore’s group at UC Davis in 2000 to do a 2nd postdoc on Arabidopsis disease resistance. In 2002, Blake started his
own research group at the University of Delaware. He was the chair of the Department of Plant & Soil Sciences at the University of Delaware from 2009 to 2015.

Abstract:
In plants, 21 or 22-nt miRNAs or siRNAs typically negatively regulate target genes through mRNA cleavage or translational inhibition. Heterochromatic or Pol IV are 24-nt and function to maintain heterochromatin and silence transposons. Phased “secondary” siRNAs (phasiRNAs) are generated from mRNAs targeted by a typically 22-nt “trigger” miRNA, and are produced as either 21- or 24-mers via distinct pathways. Our prior work in maize and rice demonstrated the temporal and spatial distribution of two sets of “reproductive phasiRNAs”, which are extraordinarily enriched in the male germline of the grasses. These two sets are the 21-nt (pre-meiotic) and 24-nt (meiotic) siRNAs. Both classes are produced from long, non-coding RNAs, generated by hundreds to thousands of loci, depending on the species. These phased siRNAs show striking similarity to mammalian piRNAs in terms of their abundance, distribution, distinctive staging, and timing of accumulation, but they have independent evolutionary origins. The functions for these small RNAs in plants remain poorly characterized. I will describe our recent work investigating the functions of plant phasiRNAs and their roles in modulating traits of agronomic importance in plants, including male fertility, as well as novel applications of phasiRNAs such as those generated from transplastomic plants.