In general, craniates are sexual organisms, that is, offspring result from the union of a male and female gamete. Tunicates can reproduce both sexually and asexually. Some vertebrates, including some Cnemidophorous (whiptail) lizards, some Ambystoma (a salamander), and some Poecilia (the popular mollies found in pet stores) are parthenogenetic. That is, only females occur and eggs start to develop after chromosome duplication.
In humans, birds, and many lizards and snakes, the sex of an individual is determined at conception because of sex chromosomes.
turtles and crocs and temperature determination.
hermaphroditism in fish
- 23 families in 7 orders of teleosts
- simultaneous hermaphrodites
- sequential
- protygnous
- protandrous
Basic design of developing gonads and reproductive ducts
all gnathostomes have the capability to produce both male and female systems
genital ridge develops in mesomere on medial side of mesonephros. The testis (in males) or ovary (in females) differentiates from this tissue
rete coords develop. In males these join to form a testis canal. Effecrent ductules connect the testis canal with mesonephric tubules. The archinephric duct, therefore,provides the exit route for sperm. In females, the rete coords develop and then regress.
An oviduct ( Mullerian duct is sometimes used for the embryonic term) develops either by lengthwise splitting of the archinephric duct (in chondrichthyans and some amphibians) or by a elongated invagination of the coelomic epithelium (other vertebrates). In males, the oviducts regress. The cranial end of the oviduct maintains an opening into the coelom (which primitively may have been the anteriormost coelomic funnels connecting the nephrocoel with the coelom). This opening is the ostium tubae.
Ontogenetic and evolutionary modifications of design
cyclostomes. lack reproductive ducts. have genital pores. Eggs and sperm are released into coelom and exit coelom into the cloaca via the genital pores.
Males:
chondrichthyans. cranial (mesonephric) parts of opisthonephros lose urinary function and simply carry sperm. Caudal part of opistonephros retains urinary function and develops 1 or more accessory urinary ducts that drain into the cloaca. There is a region of opisthonephros between the repro and urinary parts that secretes seminal fluid into archinephric duct and is called Leydig's gland.
lungfish and primitive actinopterygians have primitive system with both urinary and reproductive products using the archinephric duct.
teleosts do not use mesonephric tubules and archinephric duct to pass sperm but develop a sperm duct that drains directly to the exterior of the body next to the excretory openings.
amphibians retain primitive pattern although some salamanders have evolved accessory urinary ducts.
amniotes entirely lose the urinary function of the mesonephros which becomes wholly reproductive. The efferent ductules lead to the head of the epididymis. The body and tail of the epididymis is formed from the cranial part of the archinephric duct. Hence the amniote epididymis is homologous to the cranial part of anamniote opisthonephros and archinephric duct. The rest of the archinephric duct is the ductus (vas) deferens.
Females:
primitive pattern - uterus developing as swelling in posterior part of oviduct
mammals - diverse uterine shapes. Marsupials have divided vaginas and an accessory canal.
teleosts - ovary folds on itself enclosing a bit of coelom. Eggs deposited into this. An ovarian tube that is not homologous to the oviduct grows out of ovary, fuse and exit near the anus and excretory ducts.
Other sexual structures in amniotes and especially mammals
genital tubercles - corpus cavernosum (dorsal) and clitoris
genital groove - part of urethra and vagina
genital folds - corpus spongiosum and labia minora
scrotal swellings - scrotum and labia majora
baculum - bone in the penis of some mammals. In general the size of the baculum is inversely proportional to the development of the erectile tissues.
spotted hyena - female mates and gives birth through enlarged clitoris.