Caenorhabditis elegans establishes germline versus soma by balancing inherited histone methylation.

Formation of a zygote is coupled with extensive epigenetic reprogramming to enable appropriate inheritance of histone methylation and prevent developmental delays. In Caenorhabditis elegans, this reprogramming is mediated by the H3K4me2 demethylase SPR-5 and the H3K9 methyltransferase, MET-2. In contrast, the H3K36 methyltransferase MES-4 maintains H3K36me2/3 at germline genes between generations to facilitate re-establishment of the germline. To determine whether the MES-4 germline inheritance pathway antagonizes spr-5; met-2 reprogramming, we examined the interaction between these two pathways. We found that the developmental delay of spr-5; met-2 mutant progeny is associated with ectopic H3K36me3 and the ectopic expression of MES-4-targeted germline genes in somatic tissues. Furthermore, the developmental delay is dependent upon MES-4 and the H3K4 methyltransferase, SET-2. We propose that MES-4 prevents crucial germline genes from being repressed by antagonizing maternal spr-5; met-2 reprogramming. Thus, the balance of inherited histone modifications is necessary to distinguish germline versus soma and prevent developmental delay.This article has an associated 'The people behind the papers' interview.

Hansel, Gretel, and the Consequences of Failing to Remove Histone Methylation Breadcrumbs.

Like breadcrumbs in the forest, cotranscriptionally acquired histone methylation acts as a memory of prior transcription. Because it can be retained through cell divisions, transcriptional memory allows cells to coordinate complex transcriptional programs during development. However, if not reprogrammed properly during cell fate transitions, it can also disrupt cellular identity. In this review, we discuss the consequences of failure to reprogram histone methylation during three crucial epigenetic reprogramming windows: maternal reprogramming at fertilization, embryonic stem cell (ESC) differentiation, and the continuous maintenance of cell identity in differentiated cells. In addition, we discuss how following the wrong breadcrumb trail of transcriptional memory provides a framework for understanding how heterozygous loss-of-function mutations in histone-modifying enzymes may cause severe neurodevelopmental disorders.

C. elegans Gonad Dissection and Freeze Crack for Immunofluorescence and DAPI Staining.

The C. elegans germline makes an excellent model for studying meiosis, in part due to the ease of conducting cytological analyses on dissected animals. Whole mount preparations preserve the structure of meiotic nuclei, and importantly, each gonad arm contains all stages of meiosis, organized in a temporal-spatial progression that makes it easy to identify nuclei at different stages. Adult hermaphrodites have two gonad arms, each organized as a closed tube with proliferating germline stem cells at the distal closed end and cellularized oocytes at the proximal open end, which join in the center at the uterus. Dissection releases one or both gonad arms from the body cavity, allowing the entirety of meiosis to be visualized. Here, a common protocol for immunofluorescence against a protein of interest is presented, followed by DAPI staining to mark all chromosomes. Young adults are immobilized in levamisole and quickly dissected using two syringe needles. After germline extrusion, the sample is fixed before undergoing a freeze crack in liquid nitrogen, which helps permeabilize the cuticle and other tissues. The sample can then be dehydrated in ethanol, rehydrated, and incubated with primary and secondary antibodies. DAPI is added to the sample in the mounting medium, which allows reliable visualization of DNA and makes it easy to find animals to image under a fluorescent microscope. This technique is readily adopted by those familiar with handling C. elegans after a few hours spent practicing the dissection method itself. This protocol has been taught to high-schoolers and undergraduates working in a research lab and incorporated into a course-based undergraduate research experience at a liberal arts college.

The Pipeline CURE: An Iterative Approach to Introduce All Students to Research Throughout a Biology Curriculum.

Participation in research provides personal and professional benefits for undergraduates. However, some students face institutional barriers that prevent their entry into research, particularly those from underrepresented groups who may stand to gain the most from research experiences. Course-based undergraduate research experiences (CUREs) effectively scale research availability, but many only last for a single semester, which is rarely enough time for a novice to develop proficiency. To address these challenges, we present the Pipeline CURE, a framework that integrates a single research question throughout a biology curriculum. Students are introduced to the research system - in this implementation, C. elegans epigenetics research - with their first course in the major. After revisiting the research system in several subsequent courses, students can choose to participate in an upper-level research experience. In the Pipeline, students build resilience via repeated exposure to the same research system. Its iterative, curriculum-embedded approach is flexible enough to be implemented at a range of institutions using a variety of research questions. By uniting evidence-based teaching methods with ongoing scientific research, the Pipeline CURE provides a new model for overcoming barriers to participation in undergraduate research.

Making evolution stick: using sticky notes to teach the mechanisms of evolutionary change.

Evolution and its mechanisms of action are concepts that unite all aspects of biology, but remain some of the most difficult for students to understand. To address this challenge, we designed a hands-on activity that introduces fundamental mechanisms of evolutionary change: natural selection, genetic drift, and gene flow. In small groups, students use a population of sticky notes to reveal the consequences of each mechanism on phenotype frequency. In a followup homework assignment, students then explore how changes in phenotype frequency reflect changes in allele frequency in the population. This activity is suitable for anyone learning the basics of evolution, from high-school through the undergraduate level. We have provided detailed instructions, in-class worksheets, follow-up homework, and extensions that allow the activity to be simplified or made more complex as needed. In our own classrooms, we have observed that the concrete and collaborative nature of this activity enables students to deepen their understanding of the mechanisms through which evolution occurs. We have designed this study such that, in completing this activity, we hope to offer students the opportunity to confront potential misconceptions about evolution and gain a solid foundation for future explorations in the discipline.

A Model for Epigenetic Inhibition via Transvection in the Mouse.

Transvection is broadly defined as the ability of one locus to affect its homologous locus in trans Although it was first discovered in the 1950s, there are only two known cases in mammals. Here, we report another instance of mammalian transvection induced by the Cre/LoxP system, which is widely used for conditional gene targeting in the mouse. We attempted to use the germline-expressed Vasa-Cre transgene to engineer a mouse mutation, but observe a dramatic reduction of LoxP recombination in mice that inherit an already deleted LoxP allele in trans A similar phenomenon has previously been observed with another Cre that is expressed during meiosis: Sycp-1-Cre This second example of LoxP inhibition in trans reinforces the conclusion that certain meiotically expressed Cre alleles can initiate transvection in mammals. However, unlike the previous example, we find that the inhibition of LoxP recombination is not due to DNA methylation. In addition, we demonstrate that LoxP inhibition is easily alleviated by adding an extra generation to our crossing scheme. This finding confirms that the LoxP sites are inhibited via an epigenetic mechanism, and provides a method for the use of other Cre transgenes associated with a similar LoxP inhibition event. Furthermore, the abrogation of LoxP inhibition by the simple addition of an extra generation in our crosses establishes a unique mouse system for future studies to uncover the mechanism of transvection in mammals.