遗传 进化与生态学 11 - Sex Determination
本期的内容是性别的确定。本文集的这一部分是遗传、进化与生态学 Genetics, Evolution, and Ecology. 这门课理论上建议在阅读完文集的第一部分的内容之后再开始学习，但基础不足的朋友也可以尝试阅读喔~
这一部分的主要内容均来自 Prof. Angela J. Roles 的 BIOL 200 课程，因此本文集的这一部分均不会标记为原创。但由于文本来源不清晰，UP主还是一个字一个字码出来的文章，本文禁止非授权的转载，谢谢！
Lesson 11: Sex Determination
[1] Gametes
Biological sex is all about gametes
▸Biologically, an individual’s sex indicates what type(s) of gametes it produces.
- Historically, biologists conflated sex and gender so sometimes textbooks or scientific papers still contain “gender” when the authors actually mean “gamete type produced”.
▸In eukaryotic sexual organisms, gametes are haploid and undergo syngamy to form a new diploid zygote;
▸Syngamy = the fusion of haploid cells (gametes) or nuclei to form a diploid zygote;
- Often requires that the two gametes differ in some way;
- Thus, sperm do not fuse with other sperm, nor do eggs fuse with eggs.
Isogamy versus anisogamy
▸Isogamy: gamete types are not visibly different (morphologically and/or physiologically similar); same size, shape;
- ‘iso’ means equal while ‘gamy’ comes from a Greek word for marriage, so we get ‘equal marriage’
- Mating type (a or α) may be determined by a single locus, as in the yeast Saccharomyces
▸ Anisogamy: gamete types differ in size and/or physiology. By convention,
- ‘anisos-’ means unequal, so “unequal marriage”
- Female gamete = larger/non-motile gamete, example: “egg”
- Male gamete = smaller/motile gamete, example: “sperm”
- Note that female/male gamete describes the morphology of the gamete, not what sex chromosome it contains.
[2] Yeast Mating Type
Example: isogamy in Saccharomyces cerevisiae (brewer’s yeast); Merlini et al. 2013
▸There are 2 mating types (a and α), defined by MAT locus genotype.
- The alleles encode transcription factors that regulate production of mating pheromones and pheromone receptors.
- An α individual produces α pheromones but detects a pheromones.
▸Diploids have both alleles (a/α) and do not mate. Having both alleles changes gene expression so that the cell behaves like a diploid instead of a haploid.
- Diploids don’t produce pheromones or their receptors.
- Diploids can undergo meiosis to produce haploid spores.
Yeast flexibility: mating-type switching
▸Each haploid yeast cell, while expressing a single mating type, possesses the genes to be either mating type.
▸The MAT locus allele simply determines which set of genes are expressed, those yielding α or a mating type.
▸If all yeast cells in a population are the same mating type, some switch their MAT allele to become the other mating type!
- Yeast cells have silent copies of both MAT alleles, enabling gene conversion; copying a silent allele to replace the active one at the MAT locus.
Yeast example:
▸Gene expression is what determines the phenotype of an individual.
▸Individuals are usually able to produce multiple phenotypes, depending on which of their genes are being expressed.
▸Biological sex --- gamete production --- is more flexible and variable than suggested by a simplified XX/XY kind of understanding.
- It’s not as much about which chromosomes you have, as it is what you do with what you’ve got.
- “Sex-determining” loci are generally transcription factors, turning on some gene expression pathways and turning others off.
▸Genotype is not the sole determinant of expression – environment often matters quite a bit, even to genetic sex determination!
[3] Anisogamy Variation
Anisogamy: inter-specific variation in gamete differentiation
Mechanisms of sex determination and differentiation are very diverse across the tree of life and tend to evolve rapidly.
[4] Eukaryotic Diversity
Diversity of sex determination across plants and animals
In this figure:
(GREEN) ESD = environmental sex determination
(GREY) Haplodiploidy = males are haploid, develop from unfertilized eggs
(PURPLE) Hermaphroditism = an individual produces both types of gametes
(RED/BLUE/YELLOW) GSD = genotypic sex determination
- Homomorphic sex chromosomes = the types are morphologically indistinguishable
- Heteromorphic sex chromosomes = the types are morphologically differentiated (e.g., XY or ZY)
[5] Hermaphroditism
Individuals produce both types of gametes
▸Simultaneous hermaphrodites produce both gamete types at once
- Many Angiosper flowers produce both ovules and pollen
- Lots of nematodes and flatworms possess genitalia of both functions
▸Sequential hermaphrodites produce only one gamete type at a time but are able to switch between reproductive events
- Slipper limpets begin life as males (small and young), switching to become females when they are larger and older.
- Many fish species live in groups of one male and several females. If the male dies, one of the females changes to become a male.
[6] Environmental sex determination (ESD)
Gamete production is determined by environmental conditions
▸In ESD, external cues alter gene expression to determine what kind of gonads an individual develops.
▸Many reptiles develop gonads based on the egg incubation temperature.
▸In some fish, gonad development depends on social cues (like pheromone detection).
▸Some plant species produce flowers of only one type in a given season, depending on resource availability (e.g., jack-in-the-pulpit)
[7] Genotypic sex determination (GSD)
GSD is diverse.
▸Heterogamety (a sex has different sex chromosomes)
- In birds, butterflies, and moths, ZW = female and ZZ = male.
- In mammals, Drosophila, and hemp, XX = female and XY = male.
▸In grasshoppers, XX = female and X = male (but still diploid for all other chromosomes)
▸Haplodiploidy: In social hymenoptera (bees), females are diploid and males are haploid (for all chromosomes).
▸Polygenic SD: multiple regions of the genome interact to determine threshhold trait of male/female: zebrafish (Danio rerio) are an example.
▸Cytoplasmic SD: cytoplasmic elements, such as mitochondria or intracellular parasites, determine sex; isopods are an example.
▸UV SD: in mosses, separate sexes are only found in the haploid phase (U = immotile gametes, V = motile gametes), all diploids have the genotype UV.
[8] Dosage Compensation
What are the consequences of having XX (2 copies) versus X (1 copy)? For the genes encoded on the X, how could the sex with only one X-chromosome produce enough protein?
- X-inactivation: one X chromosome forms a Barr body; for example, human females, other mammals;
- Hypotranscription: in the homogametic sex, gene transcription of the X is reduced; for example, female Caenorhabditis elegans;
- Hypertranscription: in the heterogametic sex, gene transcription of the X is increased; for example, male Drosophila;
These changes are epigenetic --- they alter chromatin state (and thus change gene expression).
