Lecture 6 (Louden) Flashcards
Platypus (outline)
Monotreme (egg laying mammal)
Males have venomous spur with neurotoxin
Evolved from 160/170m years ago from therian lineage
~250mm years ago, homeothermy evolved – lactation
Platypus sex chromosomes
Has many Y and “X” chromosomes- 5 different X-like and 5 different Y-like
Heteromorphic but no SRY gene
No homology to eutherian X-chromosomes!
X chromosome most closely matches chicken Z
(unique)
These chromosomes link during meiosis to form a chain that ensures every sperm gets a unique set of all Xs or all Ys.
Despite the similar designations, none of the platypus X chromosomes resembles the human, dog or mouse X.
The “X” chromosome has large elements of the avian Z chromosome – so bird-like.
Suggests ancestral sex determining mechanism is 350 mY old….
Suggests SRY system in mammals evolved later, and that primordial system is based on avian system
Sex determination in the platypus
These chromosomes link during meiosis to form a chain that ensures every sperm gets a unique set of all Xs or all Ys.
Despite the similar designations, none of the platypus X chromosomes resembles the human, dog or mouse X.
The “X” chromosome has large elements of the avian Z chromosome – so bird-like.
Suggests ancestral sex determining mechanism is 350 mY old….
Suggests SRY system in mammals evolved later, and that primordial system is based on avian system
Marsupials and chromosomal dosage (wallabies)
Marsupials use a chromosomal sex determination – X & Y based
Males have a scrotum and testis, female macropodids pouches and mammary glands
Marsupials and Monotremes have a bifurcated penis
The reproductive tract of marsupials is also bifurcated, with lateral vagina and pseudo-vaginal birth canal
Wallaby offspring development
Incredibly small size of marsupial offspring – largest are Macropodids
Marsupials are characterised by producing very young, undeveloped offspring
28-day pregnancy
Development between 15-28 = near-term, in utero. Rate at which they develop in utero is broadly similar to the rate at which a therian with similar body weight would develop
500mg in weight – very large compared to other marsupial species eg marsupial cat is 12mg
Events following birth (tamar wallabies)
Newborn ruptures out of yolk sack
Offspring exhibit negative geotropism (climbs upwards to pouch using front claws)
Depends crucially on maternal posture – sits hunched over on back legs for 2-3 hours whilst giving birth. Mother has to defend herself from males whilst this happens
The new-born joey has enough energy to make the journey from the urinogenital sinus to pouch approximately twice)
Phenotypic characteristics of XO marsupials
Ovaries or streak gonads Mullerian ducts present Wolffian ducts absent No penis Scrotum present
Phenotypic characteristics of XXY marsupials
Intra-abdominal testis Mullerian ducts absent Wolffian ducts present Penis developed Pouch and mammary gland No scrotum
Conclusions drawn from the phenotypic characteristics of intersex marsupials
There must therefore be genes or clusters of genes on the X chromosome which are responsible to development of female pouch and mammary tissue. ..
An explanation of the phenotypic characteristics of intersex marsupials
Presence of pouch and mammary gland is X-dose dependent.
One “X” = no pouch, BUT A SCROTUM!
Two “X” = pouch and mammary gland
Normal XY males do not produce pouches or mammary glands. No male marsupial is known to produce breast tissue – in marked contrast to eutherians.
Marsupial mammary development is chromosomally mediated – in eutherians oestrogen is the switch and mammary glands found in both sexes.
Sex determination in different types of mammaks
Eutherians – SRY switch – testis – differentiation
Marsupials – SRY switch but also dosage mechanism based on X
Monotremes – no SRY – avian like dosage based on DMRT1(?)
Birds – gene dosage – lesser role for gonadal steroids
Similarity in sex determination of all mammals
Testosterone essential for maturation and development of male germ cells
Testosterone converted to oestrogen, which is likely essential for differentiation of the brain (partic. in birds and mammals)
Testosterone essential as driver for male-like behaviour – acting on neural circuits
In puberty, testosterone levels rise and act on primed structure in brain, act on androgen receptors instead of oestrogen, to drive male behaviour
Sex determination in reptiles
Temperature dependent, sex produced by high and low temperatures depends on species (diagram)
Sex ratio and temperature determination in alligators
Flip early – no change in sex ratio
Flip late – (low temperatures) all offspring that hatch are female
Flip between 21-28 days → significant number of males
Only ~1.5-2oC temperature range
Site of nest production in alligators
Nests are made by the females and favoured sites are nearest to water (thus tend to be slightly cooler). Therefore, alligators that get the best sites tend to produce higher numbers of females.
Nest temperatures – ones on the edge are cooler than nests in the middle
In all species exhibiting TSD there is a sensitive period at mid-development where temperature change can induce sex-reversal
