Carl Hjelmen

Postdoctoral Research Associate · Blackmon Lab · Department of Biology · Texas A&M University

Evolutionary Biologist Investigating Dynamics of Genome Size Evolution and Sex Chromosome Differentiation

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Genome Size Evolution in Insects

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 and Heterochromatin Content of the Genome

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 Chromosome Evolution

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.

Education & Experience

Postdoctoral Research Associate

Texas A&MDepartment of Biology, Texas A&M University, College Station, TX
Studying karyotype and genome size evolution across insects.

Advised by Heath Blackmon

January 2019 - Present

Postdoctoral Research Associate

Texas A&MDepartment of Entomology, Texas A&M University, College Station, TX
Proteomic and miRNA markers of fly development for applications in Forensic Entomology

Advised by Aaron Tarone

September 2017 - December 2018

Ph.D. in Entomology

Texas A&MTexas A&M University, College Station, TX
Dissertation Title: Phylogenetic analyses of genome size evolution in Drosophilidae

Advised by J. Spencer Johnston

August 2013 - August 2017

Bachelor of Arts in Biology

AugustanaAugustana College, Sioux Falls, SD
August 2009 - May 2013


Check out my Google Scholar Page


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


Honors and Awards in the News


First place PhD Oral Presentation at Southwestern Branch of ESA Meeting

2017 Outstanding PhD Student


Our Linnaean Teams win First and Second Place at Southwestern Branch Meeting!

2016 Outstanding PhD Student


First place Oral Presentation in Session at National ESA Meeting

First place Oral Presentation for Ecological Integration Symposium and Student Research Week


Check out my instagram page to see more!

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!

Pictures from the Lab and Fieldwork

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Digging for Dung Beetles
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Chrysina from our Trip
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Light Collecting
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Meeting with Aliens in Roswell
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Late Night Breakfast with the Lab
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Large cerambycid
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Lab Picture