Evolutionary Biologist Investigating Dynamics of Genome Size Evolution and Sex Chromosome Differentiation
The amount of DNA in organisms varies widely from species to species (Up to 7,000 fold in animals!). However, more DNA does not make an organism more complex. Most of this variation in DNA content is actually due to highly repetetive non-coding regions and other things like transposable elements. So why does so much variation exists? How does this variation come to be? Are there adaptive patterns to any of this change? Are there consistent patterns of genome size change that exist between organisms? I use new and old genome size estimates in conjunction with comparative phylogenetic approaches to investigate these patterns of genome size change across a range of organisms. While most of my work has been in Drosophila species, I have been part of collaborative efforts to investigate these patterns of change across multiple families of beetles as well as investigating correlations of genome size to other phenotypic correlates, such as reproductive fitness, body size, and development time. I am working to sequence individuals that have unique patterns of change in order to investigate which mechanisms are driving these significant changes in size.
Underreplication is a fascinating phenomenon by which DNA replication is stalled before replicating the late replicating heterochromatin. This phenomenon has been noted for decades in the polytene salivary glands of Drosophila, but only recently was documented in the thoracic tissue of D. melanogaster. We know variation in genome size is largely due to repepetive and noncoding DNA (largely heterochromatic), so we would expect that more of this "unreplicated" DNA occurs in species with larger genomes. We therefore can use Underreplication to study the dynamics of Heterochromatin and Euchromatin change throughout time. Are there dramatically different patterns of change between types of chromatin? Is this dependent on what group of species we look at? What is occuring in species that have significantly more heterochromatin? Why does this partial replication occur in thoracic tissue? And why in only Drosophila? Does it have something to due with adaptation to environments? What is the physiological benefit of more DNA in the thorax? I hope to answer these questions through long read sequencing, transcriptomics, and studies of phenotypes!
Sex chromosomes are often the first thing to which biologists attribute differences between sexes. While most of the DNA content in a genome is on the autosomes and therefore does not differ between the sexes, differences between sex chromosomes lead to highly differentiated gene expression and phenotypes between the sexes. In the an XY sex system, as the sex chromosomes differentiate, the Y chromosome becomes more and more sparse in the case of gene content and becomes highly heterochromatized and compacted. We have found that in many cases, not only does gene content decrease, but also the physical amount of DNA on the Y chromosome decreases. Hypothetically, the Y chromosome gets smaller and smaller throughout time, until it becomes so small it can be lost. Here we may see sex chromosome turnover and introduction of Neo-sex systems. So, how big can the differences between sexes become? How often are males in XY systems larger than females? Are those with larger male genomes indicative of Neo-Sex chromosomes? Beyond just looking at physical size, I am interested in seeing what content is found on neo-sex chromosomes and investigating whether or not specific chromosomes are more likely to be selected to become sex chromosomes.
Advised by Heath Blackmon
Advised by Aaron Tarone
Advised by J. Spencer Johnston
Hjelmen CE, Blackmon H, Holmes VR, Burrus CG, Johnston JS. Genome Size evolution differs between Drosophila subgenera with striking differences in male and female genome size in Sophophora. accepted to g3 PDF
Hjelmen CE, Garrett M, Holmes VR, Mynes M, Piron E, and Johnston JS. Genome size evolution within and between the sexes. Journal of Heredity PDF
Johnston JS, Bernardini A, Hjelmen CE. "Genome Size Estimation and Quantitative Cytogenetics in Insects." in Insect Genomics: Methods and Protocols. PDF
Lower S Sander, Johnston JS, Stanger-Hall K, Hjelmen CE, Hanrahan SJ, Korunes K, and Hall D. Genome Size in North American fireflies: Substantial variation likely driven by neutral processes. GBE. PDF
Hjelmen CE, and Johnston JS. The mode and tempo of genome size evolution in the subgenus Sophophora. PLOS One. PDF
Arnqvist G, Sayadi A, Immonen E, Hotzy C, Rankin D, Tuda M, Hjelmen CE, and Johnston JS. Genome size correlates with reproductive fitness in seed beetles. Proc. R. Soc. B. PDF
Rangel J, Straus K, Seedorf K, Hjelmen CE, Johnston JS. Endopolyploidy changes with age-related polyethism in the honey bee, Apis mellifera. PLOS One. PDF
Ellis LL, Huang W, Quinn AM, Ahuja A, Alfrejd B, Gomez FE, Hjelmen CE, Moore KL, Mackay TFC, Johnston JS, and Tarone AM. Intrapopulation genome size variation in D. melanogaster Genetic Reference Panel lines. PLOS Genetics. PDF
The DGRP Consoritum. Natural variation in genome architecture among 205 Drosophila melanogaster Genetic Reference Panel lines. Genome Research. PDF
Larson MK, Tormoen GW, Weaver LJ, Luepke KJ, Patel IA, Hjelmen CE , Ensz NM, McComas LS, McCarty OJ. Exogenous modification of platelet membranes with the omega-3 fatty acids EPA and DHA reduces platelet procoagulant activity and thrombus formation. Am. J. Physiol. Cell Physiol. PDF
Outside of the lab I like to try to fill my time with hobbies, such as:
Playing music: I'm a classically trained bassist and self-taught guitarist.
Listening to music: I'm a big fan of classical music and hard rock.
Woodworking with my Ph.D. advisor.
Hiking (when I can get out of town). and
Riding my motorcycle. It's not a small one...(1600 Vulcan Classic, for those of you that care) And don't worry, I always wear my helmet!