Reproduction

1. Reproduction The purpose of life!!

2. Sex or no sex? Sexual reproduction maintains genetic variation within a population. Tunicates both asexual (budding) and sexual. Some vertebrate species are all female – PARTHENOGENESIS e.g. whip-tailed lizards, salamander, Amazon molly. MALE OR FEMALE? Sex determination in birds & mammals genetic at time of fertilisation – GENOTYPIC SEX DETERMINATION. Sex chromosomes – male mammals are the HETEROGAMETIC SEX, sperm are either X or Y. Female mammal eggs are only X. Birds – females are heterogametic sex – WZ, males ZZ Turtles, many lizards and gators – TEMPERATURE-DEPENDENT SEX DETERMINATION. >30oC female, <25oC males. How – estrogen overrides temp. Other reptiles that retain young in uterus have genotypic sex determination.

3. HERMAPHRODITISM – evolves when genetic determination not present. Synchronous hermaphrodites – testis and ovaries function concurrently, or have OVOTESTIS – some teleosts, sea bass. Sequential hermaphrodites – many teleosts – first female then male PROTOGYNOUS; first male then female PROTANDROUS.

4. Social reversal in coral reef fish - the male in each harem suppresses the tendency of the females to change sex by actively dominating them. Death of the male releases this suppression and the dominant female of the harem changes sex immediately.

5. PATTERNS OF REPRODUCTION OVIPAROUS – fish, amphibians, reptiles. Sperm poured over eggs – cod = many millions per season, toads up to 30,000 eggs. Mortality high. VIVIPAROUS – chondrichthyians, some teleosts, caecilians, few frogs and salamanders, all mammals. Internal fertilisation (so need a penis), eggs retained in specialised part of oviduct = UTERUS. Fewer eggs needed, greater chance of survival. Microlecithal Mesolecithal Macrolecithal

6. DEVELOPMENT OF REPRODUCTIVE SYSTEM Initially the male and female systems are the same = INDIFFERENT PHASE. Begins adjacent to mesonephros (INTERMEDIATE MESODERM) and a genital ridge appears and bulges into coelom as primordial gem cells migrate from yolk sac

7. Primordial gem cells invade from yolk sac to region of mesonephros Their appearance stimulates the coelomic epithelium to thicken (germinal epithelium) and proliferate and generate primary sex cords

8. Another duct forms besides the archinecphric duct (mesonephric duct) = paramesonephric duct (Mullerian duct) here called oviduct Indifferent stage

9. MALE – SRY gene on the Y chromosome kicks in (without this embryos become female – default state). Primary sex cords expand and become seminiferous tubules of testis. Oviduct (paramesonephric duct, Mullerian duct) regresses. Archinephric duct (mesonephric duct becomes spermatic duct (vas deferens) Cells of mesenchyme separate off cords from epithelium = tunica albuginea, and some become interstitial cells (Leydig cells) and eventually generate testosterone.

10. FEMALE – no SRY gene so becomes female. Primordial gem cells carried into middle by secondary sex cords, break up into follicles with one gem cell in it surrounded by follicular cells. Archinephric duct (mesonephric duct) degenerates and oviduct (paramesonephric duct, Mullerian duct) remains tp become the conducting tube of the female.

11. FEMALE MALE Mesonephric duct (archinephric) Paramesonephric duct (Mullerian duct)

12. MATURE TESTIS Seminiferous tubules Rete testis Epididymis

13. Interstitial cells = Leydig cells, secrete testosterone Spermatogonia and Sertoli cells, contribute to nutrition and maturation of sperm and remove cytoplasmic contents. Spermatogonia primary spermatocytes secondary spermatocytes spermatids spermatozoa Spermatogenesis 64 days in humans

14. Male 2n Spermatogonium DNA synthesis Primary spermatocyte 4n 1st meiotic division Secondary spermatocyte 2n 2nd meiotic division Spermatids 1n 4 sperm

15. Spermatozoa

16. In most vertebrates, testes lie close to site of development, but in mammals they descend into an external pouch = SCROTUM, except for a few e.g. cetaceans, elephants, edentates

17. ANAMNIOTES – huge amounts of sperm mean very large testis filling body cavity e.g. fish. Fat bodies may form energy reserves for seasonal production. Immature gonad often has anterior part resembling ovary, posterior testis. Only one part functions e.g. Bidder’s organ in toads.

18. MATURE OVARY – usually paired, fused in lampreys and teleosts, only 1 in birds (left). Mammalian ovaries are smaller, eggs less yolk

19. Female female 2n DNA synthesis PRIMARY OOCYTES 400,000 in humans at birth, most die, 400 ovulated 4n 1st meiotic division 2n Start again after puberty 2nd meiotic division GRAAFIAN FOLLICLE 1n 1 egg

20. Follicular cells proliferate to form zona granulosa, oocyte enlarges forms an envelope = zona pellucida, connective tissue forms a theca Primary oocytes surrounded by follicular cells = primordial follicles found at edge of ovary FSH theca Primordial follicle Zona pellucida Zona granulosa

21. Graafian follicle – first polar body produced, stops in 2nd meiosis Theca starts producing sex hormones; follicular fluid collects in an enlarging space

22. At ovulation oocyte breaks free, follicle ruptures, egg travels down fallopian tubes, if fertilised 2nd meiotic division completed to form 2nd polar body 2nd polar body 1st polar body

23. In non-mammals eggs larger, more of them and fill up the body cavity