User:Bioclocks2021/Christine Merlin

Christine Merlin is a French chronobiologist and an associate professor of biology at Texas A&M University. Merlin's research focuses on the underlying genetics of the monarch butterfly circadian clock and explores how circadian rhythms modulate monarch behavior and navigation.

Christine Merlin
Born	September 24, 1980 Soues, Hautes-Pyrénées, France
Nationality	French
Alma mater	Pierre and Marie Curie University
Known for	Sun compass navigation in migratory monarch butterflies
Scientific career
Fields	Chronobiology, genetics, entomology
Institutions	Texas A&M University University of Massachusetts Medical School Pierre and Marie Curie University
Website	https://www.merlinlab.org/

Background

Christine Merlin was born on September 24, 1980 in Soues, Hautes-Pyrénées, France. In high school, she became interested in circadian biology while studying moth pheromone release timing. She attended Pierre and Marie Curie University in Paris, where she received a BS in animal biology, an MS in invertebrate physiology, and finally her PhD in insect physiology in 2006. She studied circadian rhythms in moths in Versailles at the National Institute for Research on Agronomy in the lab of Emmanuelle Jacquin-Joly and Martine Maibeche-Coisne while studying for her doctorate. In 2007, she began working in the lab of Steven Reppert at the University of Massachusetts Medical School. There, she studied the migration of monarch butterflies and collaborated on a paper outlining the monarch genome.

In 2013, she became an assistant professor in the department of biology at Texas A&M University, where she works now. She has been a faculty member of the Texas A&M Center of Biological Clocks Research since 2013, of Texas A&M’s Genetics and Neuroscience interdisciplinary programs since 2014, and of the Ecology and Evolutionary Biology interdisciplinary program since 2015.

Scientific contributions

Merlin has helped publish over 20 papers during the course of her career and has garnered over 2000 citations for her work.

Insect olfactory receptors and chemosensory

Olfactory receptors (ORs) can provide information on the molecular basis of chemical signal recognition in insects. Merlin and colleagues reviewed data on Drosophila melanogaster ORs to understand the olfactory mechanism of insects in relation to vertebrates. It was found that subtypes of olfactory receptor neurons express one OR type, olfactory neurons express a single type of OR, and all olfactory neurons converge at the glomerulus in the antennal lobe. Merlin and colleagues concluded that insects and vertebrates have a conserved olfactory mechanism and that insects such as Drosophila melanogaster can act as model organisms to better understand chemical communication mechanisms. In a subsequent paper, Merlin and coworkers analyzed the signal termination step of the olfactory process which uses a variety of odorant-degrading enzymes. The team isolated an aldehyde oxidase partial cDNA from the cabbage armyworm Mamestra brassicae and looked at which part of the chemosensory organs had expression. The results showed expression in the antennae, particularly the olfactory sensilla, suggesting that aldehyde odorant compounds (pheromones or the plant's volatiles) could be degraded by specific enzymes.

The migration of monarch butterflies

Monarch butterflies use the sun as a compass to precisely orient themselves across long distances during migration. Merlin and colleagues determined that circadian clocks located in the antennae play a significant role in navigation in migratory monarch butterflies. Using tests of necessity, she analyzed butterfly orientation with and without antennae and concluded that butterflies without antennae were unable to orient themselves relative to the sun. These experiments involved ablating the antennae from butterflies and coating them in black paint. She isolated butterfly antennae in vitro and discovered that they were sufficient for entrainment to light-dark cycles. By measuring expression of clock genes Per, Tim, and Cry2, Merlin determined that a circadian clock exists autonomously in the monarch antennae.

In 2011, Merlin (along with Steven Reppert, Shuai Zhan, and Jeffrey Boore) outlined the genome of the migratory monarch butterfly and described the operation of circadian clocks as a method of regulating migration. The monarch clock is a transcription-translation feedback loop (TTFL) and contains many of the same genetic components as the clocks of other arthropods, such as Drosophila melanogaster. One notable difference between the monarch clock and the Drosophila melanogaster clock is the appearance of both CRY1 and CRY2 in the monarch clock, while Drosophila melanogaster only contains CRY1. As most arthropods have both CRY types, this result provided additional evidence that both CRY were at “the base of arthropod evolution.” They highlighted two main uses for the clock. One use is the sun compass, which allows the monarch to navigate towards its destination by detecting the horizontal position of the sun and the polarized skylight patterns produced. The other use is the initiation of migration by detecting decreasing day length in the autumn.

Monarch butterflies use a time-compensated sun compass that is a part of the insect’s light-entrained circadian clocks located in the antennae. Merlin and colleagues conducted a test of sufficiency to determine if one or both antennae are needed to properly orient. They found that either antenna can be sufficient for time compensation, but surprisingly that butterflies with one of their antennae painted black and the other painted clear had disoriented flight. In this experimental condition, the antenna painted black would block entrainment from light cues while the antenna painted clear would allow entrainment from light cues. When the black-painted antenna from this experimental condition was ablated, the butterfly would then be able to properly orient by using just the single clear-painted antenna to entrain. The team also observed that Per and Tim gene expression was highly rhythmic in the clear-painted antennae, but disrupted in the black-painted antennae. Merlin and coworkers concluded that each antenna has its own clock outputs that are then processed together in the sun compass to direct the orientation of flight.

Noctuid antennal transcriptome and odor detection genes

Nocturnal insects are good subjects to study the molecular basis of olfaction and odor detection. Identifying how an odor can cause a neuronal response in an insect is very important for understanding the physiological basis of olfaction, as well as provide a possible foundation for olfactory based control strategies against moth pests. In this paper, Merlin and her colleagues aimed to determine the antennal transcriptome as well as identify possible genes that are involved with olfaction and odor detection using the noctuid pest model, Spodoptera littoralis. While characterizing the genome, Merlin and her colleagues determined that there were 6738 coding regions among the 9033 unigenes, and 90% of these coding regions showed similarity to previously coded proteins when compared to the D. melanogaster and B. mori proteomes. However, 678 of these ORFs showed no resemblance, and these ORFs were analyzed, which revealed one sequence that appeared as a new original candidate for an odorant-binding protein (OBP). Merlin and her colleagues also determined there were a large number of potential candidate genes involved with odor detection and olfaction, as well as 31 candidate chemosensory receptors on the antennae. The results from this project has allowed Merlin and her colleagues to determine several chemosensory receptors in a species that has no prior genomic data, as well as providing resources for further research into the exact mechanisms of odor detection in S. littoralis and for identifying targets to fight against herbivorous moth pests.

Genetic editing: TALENs & CRISPR/Cas9

Merlin and her lab have been consistently interested in exploring methods of selective genetic editing via tools such as zinc finger proteins, which enable the creation of targeted gene knockouts within a specified locus. In 2016, Merlin and colleagues demonstrated that both TALENs and CRISPR/Cas9 machinery could be utilized in a similar manner, to engender highly efficient, heritable, targeted mutagenesis at selected genomic loci. Merlin and her team were able to generate genetic knockouts of the Cry2 and Clk genes within the monarch genome, two notable clock genes responsible in part for the regulation and modulation of the monarch circadian clock. These knockouts were shown to be heritable, with the injection of less than 100 eggs being sufficient to recover mutant progeny; enabling the generation of entire mutant knockout lines in around 3 months. These findings provided new research methods for the study and mutation of monarch clock genes, as is currently being explored by the Merlin Lab.

Monarch migration and genetics

While researching monarchs, Merlin headed a paper alongside Samantha E. Liams and Aldrin B. Lugena and delved into a deeper understanding of monarch migration and the genetic components that are involved. By comparing the migrating monarchs of North America and Australia to the non-migrating monarchs of South America and Africa, they concluded that there is an epigenetic component that leads to migrations of these specific subsets of butterflies. Studying these populations against each other through population genomics revealed a small difference of about 2% of the genome targets of divergent natural selection, explaining how the two types of monarchs can exhibit differing migratory behaviors. Those monarchs who migrate experience a diapause response to prepare for their migration, which is triggered by the shortened days and colder temperatures of autumn. Female monarchs release less mature oocytes during this period due to clock genes, which were studied to find differences in monarchs with long and short photoperiods. The vitamin A pathway, regulated in a photoperiod manner, was credited to be different between the two groups. Using a CRISPR/Cas9-mediated loss of function mutant of gene nina B1, a gene that encodes a rate limiting enzyme that converts β-carotene into retinal, it was discovered that the ability of the monarchs under the short photoperiod were unable to enter diapause. Diapause in monarchs is caused by a juvenile hormone deficiency in the corpora cardiaca-corpora allata complex, but the link between the hormone and Vitamin A is currently unknown.

Ongoing research

The Merlin lab is currently focused on further unraveling monarch circadian genetics, utilizing reverse genetic tools to determine how previously identified candidate genes contribute to butterfly migration and the larger circadian cycle of the monarch. The lab is currently working with known clock genes in vivo to understand circadian repressive mechanisms within the monarch and also gain further knowledge regarding how insect clocks have evolved over the ages. Merlin is also exploring means to modify selected genes via genetic knock-out and knock-in methods. Utilizing CRISPR/Cas9 and other genetic editing machinery, Merlin aims to improve the efficiency of NHEJ mediated targeting, and furthermore develop a reliable knock-in approach to introduce reporter tags into loci of interest within the monarch genome.

Timeline of selected publications

• Jacquin-Joly E and Merlin C (2004) Insect olfactory receptors: contributions of molecular biology to chemical ecology. J Chem Ecol. 30: 2359-97.
• Merlin C, François M-C, Queguiner I, Maïbèche-Coisne M and Jacquin-Joly E (2006) Evidence for a putative antennal clock in Mamestra brassicae: molecular cloning and characterization of two clock genes-period and cryptochrome- in antennae. Insect Mol Biol 15: 137-145.
• Merlin C, Gegear RJ and Reppert SM (2009) Antennal circadian clocks coordinate sun compass orientation in migratory monarch butterflies. Science 325: 1700-1704.
• Reppert SM, Gegear RJ and Merlin C (2010) Navigational mechanisms of migrating monarch butterflies. Trends Neurosci. 33: 399-406.
• Zhan S, Merlin C, Boore JL and Reppert SM (2011) The monarch butterfly genome yields insights into long-distance migration. Cell 147: 1171-1185.
• Merlin C*, Beaver LE, Taylor OR, Wolfe SA and Reppert SM* (2013) Efficient targeted mutagenesis in the monarch butterfly using Zinc Finger Nucleases. Genome Res 23:159-68.
• Markert MJ, Zhang Y, Enuameh MS, Reppert SM, Wolfe SA and Merlin C (2016) Genomic Access to Monarch Migration Using TALEN and CRISPR/Cas9-Mediated Targeted Mutagenesis. G3: Genes, Genomes, Genetics. doi: 10.1534/g3.116.027029.
• Merlin C, Liedvogel M, el Jundi B, Kelber A, Webb B (2019) The genetics and epigenetics of animal migration and orientation: birds, butterflies and beyond. J Exp Biol 222 (Suppl_1): jeb191890. doi: https://doi.org/10.1242/jeb.191890

Collaborators

Throughout her career, Merlin has worked with many other chronobiologists. She performed her graduate research in Emmanuelle Jacquin-Joly and Martine Maibeche-Coisne’s lab in Versailles and carried out her post-doctoral training in Steven Reppert’s lab. Other notable co-authors include Marie-Christine François, Robert J. Gegear, Ying Zhang, Sébastien Malpel, Scot A. Wolfe, Patrick Porcheron, and Patrick A. Guerra.

Honors and awards

The Merlin Lab receives funding from the National Science Foundation, the National Institutes of Health, and the Esther A. & Joseph Klingenstein Fund. Merlin has received the following awards for her work in circadian biology:

• Charles King Trust Postdoctoral Fellowship from The Medical Foundation (2012)
• Klingenstein-Simons Fellowship Award in Neuroscience (2017)
• Junior Faculty Research Award from the International Society for Research on Biological Rhythms (2018)
• Presidential Impact Fellowship at Texas A&M University (2020)

See also

• Sun compass in animals
• Circadian clock
• Monarch butterfly migration