本期的內(nèi)容是性別的確定。本文集的這一部分是遺傳、進化與生態(tài)學(xué) Genetics, Evolution, and Ecology. 這門課理論上建議在閱讀完文集的第一部分的內(nèi)容之后再開始學(xué)習(xí)，但基礎(chǔ)不足的朋友也可以嘗試閱讀喔~

這一部分的主要內(nèi)容均來自 Prof. Angela J. Roles 的 BIOL 200 課程，因此本文集的這一部分均不會標(biāo)記為原創(chuàng)。但由于文本來源不清晰，UP主還是一個字一個字碼出來的文章，本文禁止非授權(quán)的轉(zhuǎn)載，謝謝！

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.

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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,

????-?‘a(chǎn)nisos-’ 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.

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[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.

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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!

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[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.

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[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)

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[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.

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[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.

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[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).