Lecture Summaries

1 Introduction

In this session, we will introduce ourselves and review the course syllabus and requirements. In addition, we will introduce and discuss some important components of the scientific writing and review process, including the Pubmed literature database and peer review, which are important for completing the midterm and final assignments. Finally, we will introduce sperm morphology and diversity by playing a rousing game of "guess the sperm."

Guess the Sperm (PDF)

Guess the Sperm Answer Key (PDF)

2 Epidemiology of Sperm Counts In 1992, a Danish study reported that sperm counts had declined globally by 50% between 1938 and 1991. This study garnered a lot of media attention and suggested that environmental changes, such as increased exposure to endocrine disruptors like BPA, are threatening the survival of our species. However, many scientists subsequently criticized the results of this study and argued that it did not consider other variables that might be associated with sperm counts (e.g. age, geographic location or differences in sperm count measurement). We will cover this original Danish study and then discuss a paper that re-analyzes the first study's data and comes to a different conclusion after accounting for differences in the geographical location of participants. This class will highlight issues of confounding and bias in epidemiological studies.
3 Spermatogenic Cycle This week we will discuss two papers about spermatogenesis, which is the process of making sperm from undifferentiated germ cells. In mice, this process is tightly regulated both spatially and temporally. Each undifferentiated germ cell begins spermatogenesis every 8.6 days, producing four mature haploid sperm 35 days later. However, the timing of different germ cell populations is staggered so that there is a continuous production of sperm within the seminiferous tubule. The first paper illustrates the importance of Vitamin A during spermatogenesis and shows how Vitamin A can synchronize the spermatogenic cycle throughout the testis. The second paper demonstrates that the timing of the spermatogenic cycle is intrinsic to germ cells in that the period of their cycle is retained when they are transplanted to the testis of a different species.
4 Male-biased Mutation Rates Germline mutations are the fuel for evolutionary change and underlie heritable disorders. Most of these mutations are thought to result from errors during DNA replication. Interestingly, it has been observed across many species that males have a higher mutation rate than females and thus contribute disproportionately to heritable disease and evolution. This sex difference is hypothesized to result from the higher number of cell divisions involved in sperm development and the corresponding increase in rounds of DNA replication. Furthermore, since males continuously produce sperm throughout their lifetime, increases in mutation rate with age will occur only in males (called the paternal age effect). This week we will read about two approaches to estimate sex-specific mutation rates. The first paper examines patterns of sequence divergence in sex chromosome-encoded genes to find that genes that male-specific genes (i.e. genes on the Y and W chromosomes) have higher mutation rates. The second paper sequences a large pedigree of chimpanzees and finds a seven-to-eight fold higher mutation rate in males and a stronger paternal age effect in chimpanzees than in humans.
5 Germ Line Selection Natural selection is usually studied at the level of the organism: An individual is more likely to survive and reproduce if it has a beneficial heritable trait. This process also takes place at the level of the sperm: One sperm might be more or less likely to fertilize an egg because of a given heritable trait. For example, mutations in the FGFR2 gene confer a growth advantage on sperm progenitor cells, increasing the probability that an FGFR2-mutant sperm will fertilize an egg. However, what is good for the sperm is not always good for the embryo, and FGFR2 mutations can cause developmental defects, including webbing of the hands and feet. In this class, we will discuss two examples of this kind of germ line selection, while comparing the beneficial and deleterious effects on the germ cells and the offspring. The first paper examines patients with Fragile X syndrome, which is caused by expansion of a three-nucleotide repeat sequence. The authors ask why the sperm of Fragile X patients, unlike the rest of the cells in their bodies, do not contain a fully expanded Fragile X mutation. In the second paper, the authors provide three sources of evidence that sperm carrying an FGFR2 mutation gain an advantage over other sperm.
6 Meiotic Recombination Meiotic recombination is a key step in spermatogenesis and involves the exchange of DNA segments between homologous chromosomes. This process generates new combinations of alleles and ensures proper segregation of chromosomes. Recent work has demonstrated that sites of recombination vary along chromosomes and are concentrated at specific regions, referred to as recombination "hotspots." These locations differ substantially between species and might contribute to the fertility problems observed between interspecies hybrids. This week we will explore how the locations of recombination hotspots are specified. The first paper shows that targeting of the Spo11 protein to DNA is sufficient to initiate meiotic recombination in yeast, and the second paper demonstrates that the binding of the PRDM9 protein influences the locations of hotspots in mice. At the end of class, we will use the results from these two papers to discuss a unified model of recombination hotspot specification.
7 Field trip to the Whitehead Institute Class will meet on the fourth floor of the Whitehead Institute, where we will prepare and examine sperm samples first-hand. We will examine slides containing fixed sperm from several species, compare their morphologies, and discuss how differences in morphology might relate to sperm function. Then we will prepare and stain slides of our own with live mouse sperm samples.
8 Chromosome Segregation Meiosis is a major cellular event in sperm development. During meiosis, chromosomes replicate, the nuclear envelope breaks down, homologous chromosomes pair and undergo recombination, and the germ cell divides twice to produce four haploid cells. Together, these events require a major reorganization of the nucleus and of the entire cell. This process must be finely coordinated, and as a result it represents a point of vulnerability for the developing sperm. In this class, we will discuss two papers that highlight how meiosis can impact non-meiotic aspects of sperm function. The first paper discusses the relationship between the meiotic spindle and the sperm flagella, a structure generated much later during sperm development. The second paper focuses on hybrid sterility—the reason a mule is infertile—and the consequences of having genomes from two different species participate in meiotic division.
9 Review Midterms This week students will have the opportunity to experience the peer review process first-hand. Each student will be assigned to lead a discussion of one of their classmate's midterms. This exercise is meant to be a fun and interactive introduction to the peer review process; the outcome of the discussion will not affect midterm grades.
10 Transcription and Chromatin State During Spermatogenesis Spermatogenic cells express more protein-coding genes and more unique mRNA isoforms than any other cell type. In general, sperm seem to play by slightly different rules when it comes to transcriptional regulation. In this class, we will discuss some of these different "rules" and their consequences for evolution. In the first paper, the authors seek to understand the origins of new genes during evolution. Using Drosophila species as a model system, they find that new genes tend to be expressed specifically in the testis. The second paper proposes a possible mechanism for this effect: the authors compare gene expression in the testes of several mammalian species using genome-wide deep sequencing approaches and find widespread transcriptional activity in spermatogenic cells at both conserved coding sequences and poorly-conserved intergenic sequences, providing a possible substrate for new gene birth. In addition to learning about sperm-specific gene regulation across species, we will use this paper to discuss methods and interpretation of genome-wide sequencing data.
11 Adaptive Evolution of Sperm Proteins Sperm from different species are morphologically diverse and, at the molecular level, sperm proteins evolve more rapidly than proteins expressed in other tissues. This week we will learn about the molecular evolution of sperm proteins and about the methods used to detect positive selection acting on them. The first paper provides evidence that positive selection acts on protamines, which are small proteins that replace histones during spermatogenesis. The second paper analyzes the sequence evolution of a diverse range of proteins and finds that sperm proteins evolve more rapidly than those from other tissues. For both papers, we will explore hypotheses for why sperm proteins evolve so quickly.
12 Sperm Cooperation In some species, a single female will mate with several males within a short interval, such that sperm from several individuals might be present simultaneously in her reproductive tract. Sperm bundling is one mechanism that has evolved as a result of this inter-individual competition. In sperm bundling, individual sperm physically latch on to one another and swim together, resulting in faster swimming velocity. This mechanism is hypothesized to have evolved to allow sperm from one male to out-compete sperm from other males. In this class, we will discuss two papers focused on this phenomenon in wild mice. The first paper characterizes sperm bundling and lays out a hypothesis about its function. The second, published eight years later, adds critical evidence to support that hypothesis. We will use this pair of papers to discuss how evidence can accumulate over several publications, and how new evidence can alter a hypothesis over time.
13 Sperm Competition Earlier, we discussed sperm cooperation, which likely evolved because of sperm competition. This week we will discuss further evidence for sperm competition, and its consequences for sperm biology. From the first paper, we will confront the "killer sperm" theory, which, like sperm cooperation, involves altruism between sperm. In the second paper, the authors devise an experimental system to test the consequences of removing sperm competition. They find that some of their predictions are confirmed while others are not. In addition to sperm competition, these papers will also provide a context for discussion of negative results and for how popular media attention relates to the scientific literature.
14 Student Presentations This will be our last class. Each student will spend 15 minutes presenting a paper related to one of the topics in this course (see description above). We will conclude with a review of the course and discuss any remaining questions.