W3L3 Thu marsupial reproduction

Question 1

General characteristics of marsupials

Answer 1

• many features similar to eutherians
• diversity of reproductive patterns and life history strategies
• females:
  – distinctive reproductive tract
  – many have a pouch
  – placenta
  – give birth to altricial young
• males:
  – scrotum is anterior to penis

Question 2

The pouch

Answer 2

some marsupial + monotreme species
Ø Thought to be supported by epipubic bones (specific to marsupials)
Ø Development is independent of hormones (XX = pouch)
Ø During lactation + development of young, pouch develops to accommodate increasing size of young
Ø Pouch primordia present in male pouch young of some marsupial species but don’t develop

Question 2

Female reproductive tract of marsupial

Answer 2

§ 2 independent uteri each with its own cervix
Ø Gravid uterus where fertilisation occurs (foetus develops)
§ 3 vaginas (2 lateral, 1 median):
Ø Median vagina: where young travels during birth, permanent structure in kangaroos + wallabies, connective tissue that forms before birth in other marsupials
Ø Lateral vaginas: where sperm travels to reach uterus
§ Ureters travel b/w median + lateral vaginas to bladder (prevents fusion of Mullerian ducts)
§ Urogenital sinus: common opening of urogenital system + reproductive tract

Question 2

Male Reproductive Tract

Answer 2

Bifid penis in many species to ejaculate sperm into both lateral vaginas
§ Scrotum is anterior to penis
§ Large prostate, Cowper’s glands, epididymis, vas deferens

Question 3

Altricial young (birds & mammals)

Answer 3

Altricial – “requiring nourishment”
-Young that are incapable of moving around on their own soon after being born.
-They are born helpless and require care for a comparatively long time.
-tammar wallaby fetus, day of birth (~0.4 g, ~15 mm long)
-Neonate vs. maternal weight is mostly proportional (except quoll)

Question 4

Tamar Wallaby

Answer 4

Tammar wallaby: Australian macropodid, monovular, highly seasonal breeders
§ As day length increases (Dec), a blastocyst comes out of diapause (embryo reactivation) + develops until birth 25-28 days later
§ After birth (Jan), female mates again, zygote develops into a blastocyst + remains in diapause in uterus until embryo reactivation

Question 5

Tamar wallaby diapause lactation control

Answer 5

suckling on mammary gland → stimulate afferent arc → pass through the spinal cord → inhibits dopamine → increase prolactin production→ stops development of corpus luteum
Ø No uterine secretions – blastocyst in diapause
Ø If young dies/no suckling: ↓prolactin → CL develops → progesterone → stimulates uterus → activation of blastocyst →develop + born a few weeks later

Question 6

Tamar wallaby seasonal control

Answer 6

Seasonal control: ↓sunlight → SCN → SCG → pineal gland → ↑melatonin → inhibits dopamine → ↑prolactin → keeps blastocyst quiescent
Ø Ensures young are born in a time where nutrients are plentiful
* Embryo reactivation: ↑sunlight → ↓melatonin → ↑dopamine → ↓prolactin → ↑progesterone

Question 7

Prolactin secreation and role

Answer 7

• Prolactin secretion ↑during lactation + in non-breeding season
  § promotes mammary development + milk secretion, inhibits corpus luteum development (prolactin suppresses progesterone → prevents uterine development → leads to diapause)
  § Suppression of prolactin: during diapause leads to reactivated CL, uterine secretions + embryo reactivation

Question 8

What is Diapause

Answer 8

Fertilised egg covered by mucoid coat + shell coat secreted by reproductive tract (marsupials)
§ Embryo develops to blastocyst stage ~100 cells + enters diapause (10 days)

Question 9

Pre-natal tammar development

Answer 9

Embryonic development (gastrulation) + foetal development (limb formation)
§ Only developed enough to get to teat (forearms, olfactory system)

Question 10

Newborn marsupial development

Answer 10

Functional mesonephric kidney for 1st week after birth
§ Gut is immature but functional to digest milk
§ Permanently attached to teat 3-4 months (unable to reattach if detached)
§ Initially ectothermic (heat from mother) + becomes endothermic (control body temp)
§ Born with pouch/scrotum but testes/ovaries differentiate a few days after birth

Question 11

sexual Development of wallaby

Answer 11

-Males: prostate buds form 30-40 days, phallus (penis) forms 100 days, pubertal 18-24 months
§ Females: pubertal ~280 days

Question 12

lactation phase

Answer 12

sophisticated process, composition of milk changes to reflect development
Phase 1
§ Mother prepares for birth + lactation for offspring
§ Development of mammary gland, secrete milk due to prolactin
Phase 2a
§ Young continuously attached to teat
§ Milk composition: high carbohydrate, low lipid (less viscous milk, easily digested, low energy requirement – small, ectothermic, little growth)
Phase 2b
§ Young intermittently attached to teat
§ Milk composition: high carbohydrate, low lipid
Phase 3
§ Young weaned + out of pouch eating grass
§ Milk composition: low carbohydrate, high lipid (gut has developed, endothermic change, growth)

Question 13

Why are marsupials good models
for sexual differentiation studies?

Answer 13

• altricial young: post-natal development is equivalent to the intra-uterine development of eutherians
• young accessible in pouch during developmental stages that are only accessible in utero in other mammals

Question 14

Sexual dimorphism in the tammar neonate

Answer 14

• gonads undifferentiated at birth
• scrotum or mammary primordia present at birth; thus sexual dimorphism occurs independently of testicular hormones
  -Pouch/scrotum development depends on no. of X chromosomes, not sex determination region (Vincent XXY no scrotum, pouch, penis)

Question 15

Virilisation in Humans and Wallabies

Answer 15

development of male physical characteristics as a result of excess androgen production
§ Humans: secondary sexual characteristics develop synchronously during early development
Ø In early development, different levels of testosterone from gonads circulating b/w males + females
Ø Leydig cells form → testosterone secretion → prostate/epididymis/vas deferens/penis/scrotum form
Ø Classical pathway: progesterone → testosterone → dihydrotestosterone
§ Tamar pouch young: secondary sexual characteristics develops sequentially
Ø In early development, testosterone levels identical b/w males + females
Ø Scrotum is present before Leydig cells produce testosterone
Ø Alternate pathway: progesterone → androstanediol → dihydrotestosterone → male sexual characteristics in early development

Question 16

Fat-tailed dunnart: new marsupial model suitable for genetic manipulation

Answer 16

Deriving spermatogonial SCs, iPSCs, ESCs for genetic manipulation
§ Goal: engineer cane toad toxin resistance in northern quolls