Evolution Atlas
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Chapter 27: The Reproductive System Evolution Atlas |
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The Reproductive System
by Brian Wisenden
The main functions of the reproductive system are to 1) reproduce, and 2) produce genetically diverse offspring. The second function is responsible for the evolution of sex and gender. Two distinct forms (female and male) produce haploid cells that fuse to form a diploid zygote. Recombining genetic material creates tremendous diversity. Diversity is the only defense over evolutionary time against a changing environment. Sexual individuals that reproduce in a way that produces diverse offspring is much more likely to be genetically represented in future generations than asexual individuals that reproduce genetically identical offspring by mitosis.
How are the sexes determined? Humans (and all mammals) possess sex chromosomes, X and Y. Females are homogametic (i.e., homozygous for X: XX) and males are heterogametic (i.e., heterozygous: XY). Other vertebrates use other systems. In birds, males are homozygous (ZZ) while females are heterozygous (WZ). Sex chromosomes are common in reptiles where either male or female can be the heterozygous sex, but sex chromosomes are often absent in amphibians and fish. Many reptiles use incubation temperature to determine sex rather than an intrinsic genetic mechanism. Eggs incubated above a threshold temperature become female while eggs below that temperature become male. In some species the relationship between temperature and sex is reversed. Some authors have wondered if the decline of the dinosaurs was related to changes in global climate affecting a temperature-dependent mechanism for sex determination. By that token, a constant internal body temperature in birds and mammals may have selected for the evolution of sex chromosomes.
The tripartite kidney and the co-evolution of the urinary system and the male reproductive tract
Nephric tubules (nephrons) form embryonically along the nephric ridge. The ridge comprises three regions; anterior, middle and posterior. The functional kidney uses nephrons from only one or two regions of the ridge. A pronephric kidney uses only the nephrons from the anterior region of the ridge. A mesonephric kidney uses only the middle region, a metanephric kidney uses only the posterior region and an opisthonephric kidney uses the middle and posterior regions.
Pronephric kidneys are found as a transitional kidney in fish larvae and larval amphibians. A duct, called the pronephric duct or the archinephric (=old kidney) duct join nephrons to the cloaca. Adult fish and adult amphibians have an opisthonephric kidney after metamorphosis. Mesonephric kidneys are found only as a transitional kidney in the embryos of amniotes (reptiles, birds, and mammals). The archinephric duct is renamed as the mesonephric duct. Adult amniotes use a metanephric kidney. In the metanephric kidney, a new duct is formed, the ureter, that connects the posterior region of the kidney to the cloaca. The archinephric duct is retained as the ductus deferens, or vas deferens, and is used for sperm transport.
Jawless fishes lack ducts to conduct sperm out of the body. Spermatozoa are shed directly into the body cavity and exit at the cloaca. In sharks (kardong, fig. 14.33, p559), an accessory urinary duct (not the archinephric duct) drain the posterior uriniferous kidney while the anterior kidney uses the archinephric duct. The testes connect to the archinephric duct (ductus deferens) at the anterior region of the kidney by small tubules similar to the epididymis. In jawed bony fishes, the testes develop separate ducts to the cloaca in most species. The archinephric duct drains the kidneys.
Amphibians have variable forms of sperm transport. In adult frogs, ducts from the testes bypass the anterior kidney and connect directly to the archinephric duct (Kardong p560, fig 14.35a). In adult salamanders, sperm travels from the testes through ducts in the anterior kidney to the archinephric duct while accessory urinary ducts drain the posterior kidneys (Kardong p559, fig 14.34). In forms such as the mudpuppy (Necturus) that become sexually mature while retaining the larval aquatic form (paedomorphic), the archinephric ducts transport both sperm and urine. In amniotes, the archinephric duct (ductus deferens) conducts only sperm. Some embryonic mesonephric tubules contribute to the epididymis. The metanephric tubule (=ureter) drains the posterior kidney.
The scrotum
Testes of most vertebrates are located within the body cavity. In mammals, a
variety of conditions occur. Testes remain in the body cavity in the primitive
egg-laying mammals, sea cows, elephants, sloths, dolphins, whales and armadillos.
The testes descend into a muscular pouch in moles, shrews, many rodents, rabbits,
seals and hyenas. In others, such as chipmunks, squirrels, some bats and some
primates, the testes descend into a scrotum only during the breeding season.
In most other mammals, the testes descend out of the body cavity through the
inguinal canal during embryonic development and remain permanently descended
in a scrotum. Cremaster muscles in the scrotal sac adjust temperature of the
testes by adjusting proximity of the testes to the body. On average the human
testes are about 8_ C cooler than the body core. The cooler temperature allows
for better spermatogensis. For this reason, tight pants can lead to low sperm
count. It is not clear why descended testes are not found in all mammals.
Female reproductive tract
The female reproductive tract evolved separately from the urinary system.
A second duct forms during embryonic development parallel to the mesonephric
duct called the müllerian duct. It has a funnel, shell gland, isthmus, oviduct,
and uterus.
Primitive jawless fishes lack any reproductive tracks. Mature ova are shed directly into the coelom and released out of the cloaca. In sharks, müllerian ducts form but are rudimentary (Kardong, fig 14.24a, p552). Advanced bony fishes invent a new ovarian duct (müllerian duct atrophies) derived from peritoneal folds (Kardong, fig 14.24d, p552) but this group evolved well past the evolutionary split that led to humans. Amphibians, reptiles, birds and mammals all have ovaries connected to the cloaca by a müllerian duct (Kardong, fig 14.27, p555). The müllerian duct is differentiated into regions for specialize functions as oviducts, uterus, and vagina.
The female reproductive tract of monotremes has a simple urogenital sinus leading from the cloaca to a pair of uteri. In marsupials, the uteri still comprise separate ducts, but now lead from a common vaginal sinus (Liem et al. p678, fig 21-18). Two lateral vaginas lead from the vaginal sinus and meet caudally at the urogenital sinus. The marsupial penis has a forked tip to ejaculate semen into each lateral vagina simultaneously. Placental mammals have a single, central vagina that is separated from the uterus (i) by a cervix. In rabbits, many rodents and elephants, the two uteri are separate, known as a duplex uterus (Liem et al. fig 21-20a). In most carnivores (cats, dogs, raccoons, bears) and many ungulates (cows, sheep) the two uteri show partial fusion with (bipartite uterus) or without (bicornuate uterus) a partial septum separating them (Liem et al. fig 21-20b,c). In most primates, including humans, but also armadillos, the fusion of uteri is complete resulting in a single "simplex" uterus (Liem et al. fig 21-20d). Bipartite and bicornuate uteri are well suited for multiple implantations of embryos along the length of each uterus. A simplex uterus is well suited for developing a single offspring per reproductive bout.
Fig sources
Kardong
Martini et al.
Liem et al.