Lecture 11: Sex chromosomes and dosage competition Flashcards
Who first identified the X chromosome in wasp sperm?
Who later discovered the XY pairing in beetle chromosomes?
Hermann Henking (1891)
Nettie Stevens (1905)
sex chromosomes have three other names, what are they?
- Heterosomes
- Allosomes
- Gonosomes
What is a hermaphrodite?
Species that have both male and female reproductive organs.
- some hermaphroditic species can self-fertilize while others require partners
- Some hermaphrodites are males or females at different times in their life cycle (sex reversal in clownfish, bluehead wrasse)
Often in animals, how is sex determined?
Sex determination is associated with a pair of “sex” chromosomes that genetically determine sex
How are three ways in which sex is environmentally determined in some species?
- Temperature during embryonic development (alligators, tuatara, sea turtles)
- Social organization (sex-ration) - most reef fish can change sex during their lifetime
- Some species are parthenogenic: females lay fertile eggs - no male required (walking stick insects, some fish/lizards - sharks in captivity)
What is monoecious?
Are monoecious plants bisexual or hermaphroditic?
Provide an example.
Houses both sexes (male and female reproductive organs) on the same plant but are separated
-two houses
- bisexual (NOT hermaphrodite)
- eg Banana
What is a hermaphroditic plant vs a bisexual plant?
Hermaphrodite flowers have male and female reproductive organs on the same flower
Bisexual (monoecious) plants have both male and female reproductive organs, but they are on DIFFERENT flowers
What is a dioecious plant?
Has only male or female reproductive parts.
Eg holly tree, cannabis
What is interesting about sex determination in clown fish (aka anemone fish)
Able to change sex when exposed to changing conditions
- If dominant female in a group dies, the dominant male becomes the female and the next male in line becomes the dominant male
Define simultaneous hermaphroditism
Individuals possess fully functional male and female gonads
Define sequential hermaphroditism
(give an example of an organism that displays this)
Clownfish:
- fish change sex sometime during its life (either from male to female protandry or from female to male protogyny)
What is protogyny?
Protandy?
- Protogyny: Female to male
- Protandry: Male to female
In some, particularly protandrous (M->F) species, sex change is _______ dependent and eventually every fish in the population will change sex
In some, particularly protandrous (M->F) species, sex change is size-dependent and eventually, every fish in the population will change sex
How is sex determined in alligators?
- Sex is fully determined by the time of hatching
- temperatures of 30C produce all females
- Temp of 34 produce all males
- Natural ration is 5F:1M
What is parthenogenesis?
Where a female can lay fertile eggs - don’t require a male
X-Y system is based on _________ distinct chromosomes
X-Y system is based on morphologically distinct chromosomes
In the XX-XY system, what is homogametic and heterogametic?
Homogametic - XX = female
Heterogametic - XY = male
Mammals use heteromorphic sex chromosomes. What does this mean?
The chromosomes look different (ie morphilogically distinct)
What are 3 notable mammal species that do not conform to the XY system?
Spiny rat and mole vole have lost the Y chromosome and the Sry gene
- Both males and females are X0
Akodan south American grass mice - females can be either XX or XY
How is sex determined in the X0 system?
Females will have 22 chromosomes + two X chromosomes (22+XX)
Males will have 22 + X (only one X)
How is sex determined in the ZW system?
ZZ = male
ZW=Female
Males are homogametic while females are heterogametic (opposite of XY system)
How is sex determined in a ploidy system (haplo-diploid system)?
- Females are diploid
- Males are haploid (monoploid)
- produced from unfertilized haploid eggs
What is the default sex in humans?
Female (XX)
X chromosomes have most of the genetic information
How are male humans produces?
Presence of Y chromosome
- Y chromosome has genes that encode ‘maleness’
- SRY/TDF - testis determining factor
- Gonad tissue is transformed from ovary into testes
- testosterone is produced
- turns on other male-specific genes
Follow human sex determination in the absence of a Y chromosome (5)
- No Y chromosome
- Gonads develop into ovaries
- Ovaries produce estrogen (steroid hormone)
- estrogen binds to Estrogen receptor - results in:
- Female development
Follow human sex determination in the presence of a Y chromosome (what is on the Y chromosome that results in Male)? (6)
- Y chromosome
- Has SRY gene (testis determining factor)
- embryonic gonads develop into testis
- testis produce steroid hormone testosterone
- testosterone binds to testosterone receptor (AR = androgenous receptor)
- Active AR triggers male development
What are bipotential gonads?
Produced by embryo - have the potential to go in either direction (hence bipotential)
- ie can become male or female
In the presence of SRY gene (on Y chromosome) the bipotential gonads develop into ________ cells and then into _________cells. This occurs at ~___ weeks after conception
In the presence of SRY gene (on Y chromosome) the bipotential gonads develop into sertoli cells and then into leydig cells
This occurs at ~5 weeks after conception
The sertoli cells produce _____ (hormone) which results in a degeneration of the _____ duct (which normally develops into the uterus and fallopian tubes)
The sertoli cells produce AMH (anti-mullerian hormone) (hormone) which results in a degeneration of the Mullerian duct (which normally develops into the uterus and fallopian tubes)
The _____ cells and _____ cells make up the testis which produce ______ (steroid hormone)
The sertoli cells and leydic cells make up the testis which produce testosterone (steroid hormone)
Testosterone acts on the ______ duct to stimulate the development of the epididymis and vas deferens
- when does this occur?
Testosterone acts on the wolffian duct to stimulate the development of the epididymis and vas deferens
- ~8weeks post conception
DHT (derived from testosterone) has what effect during development?
DHT = dihydrotestosterone
- stimulates the urogen and sinus to develop into the external genetilia in males
How are sex-reversals caused?
Aberrant SRY loci
- SRY+ in XX = male genitilia
- SRY- in XY = female genitilia
In which region do human x and y chromosomes pair and undergo crossover?
Psudoautosomal region
