Sexual reproduction involves the meeting of two specialized cells called gametes from each parent.
Male gametes are spermatozoa (sperm), and female gametes are ova (eggs/oocytes).
Fertilization is the point where a sperm successfully penetrates the egg.
Fertilization can occur internally (within the female) or externally (in the surrounding environment).
External Fertilization
Aquatic animals like frogs, toads, bony fish, and marine invertebrates usually release their gametes into the water, known as spawning.
Environmental factors such as temperature, photoperiod, lunar periodicity, and tidal cycles are important in triggering synchronized spawning between males and females.
Internal Fertilization
Internal fertilization typically involves a male reproductive organ inserted into the female reproductive tract (e.g., penis in mammals).
Some species, like most birds, reproduce via internal fertilization by pressing cloacas together.
Salamanders may drop a spermatophore (packet of sperm) that the female picks up with her cloaca after courtship.
Elasmobranchs transfer spermatophores using modified pelvic fins called claspers.
Many terrestrial arthropods also utilize spermatophores.
Number of Eggs, Energy, and Protein Provision
Birds, most reptiles, and terrestrial insects lay self-contained eggs called cleidoic eggs.
Egg-laying in mammals only occurs in monotremes (platypus and echidna).
Those using external fertilization tend to release a higher number of eggs (r-strategists vs. K-strategists).
Yolk Distribution and Amount
Yolk consists of nutrients and enzymes for the development of fertilized eggs; its distribution differs between species.
Isolecithal eggs have evenly distributed yolk (e.g., mammals).
Teleolecithal eggs have yolk concentrated on one side, opposite the embryo (e.g., amphibians).
Centrolecithal eggs have yolk concentrated in the center of the egg (e.g., insects).
Eggs are categorized based on the amount of yolk they contain:
Alecithal eggs have no yolk (e.g., placental mammals).
Microlecithal eggs contain small amounts of yolk (e.g., marsupials and echinoderms).
Mesolecithal eggs contain intermediate amounts of yolk (e.g., most fish and amphibians, enabling embryos to hatch at later stages).
Macrolecithal eggs have a large amount of yolk (e.g., insects, cartilaginous and bony fish, reptiles, and birds, enabling young to hatch even later).
Precocial vs. Altricial Bird Species
Precocial bird species hatch as well-developed mobile chicks with eyes open, covered with down, and able to thermoregulate.
Altricial bird species hatch with their eyes closed, incapable of moving around, and unable to thermoregulate.
Small eggs with less yolk typically hatch earlier in development.
Lipid and Protein Provision in Eggs
Lipid and protein provision within an egg has cost implications for the female.
Capital breeders accumulate sufficient reserves for egg formation and laying.
Income breeders rely on continual food intake to supply protein/lipids to the eggs.
Vitellogenesis is the process of depositing nutrients (mainly vitellogenin) into oocytes that will develop into yolky eggs.
Vitellogenins in males can be used as biomarkers for exposure to environmental estrogens and other chemical pollutants because vitellogenin is normally associated with females.
Egg-Laying Strategies
Oviparous animals release eggs with little or no embryonic development.
Ovoviviparous animals retain fertilized eggs, providing protection, but nutrients are within the eggs.
Viviparous (live-bearing) animals provide the daily nutritional requirements of the embryo throughout development.
Placental (eutherian) mammals develop a placenta, forming a connection between mother and fetus, providing nutrients, oxygen, and hormones while removing metabolic wastes and carbon dioxide.
Placenta-like structures also occur in some snakes, lizards (skinks), salamanders and frogs, and some sharks, but is never a characteristic of all species.
Most animals reproduce more than once in their lifetime (=iteroparity).
Some animals have a once-in-a-lifetime breeding event (=semelparity) and are characterized by the extent to which their body becomes dedicated to reproduction.
Seasonal Breeding Cues
Fluctuations in rainfall and changing photoperiod (altering melatonin secretion) act as cues for seasonal breeding.
Temperature also plays an important role in timing reproduction, especially in ectotherms, and can have interactive effects with photoperiod.
Variations in food intake and quality can affect reproductive success (carryover).
Genetic Sex Determination
Genetic sex determination occurs in most animal species. The most common system is the XY/XX sex chromosome system.
Genetic males are heterogametic (XY), and genetic females are homogametic (XX).
During gamete formation, chromosome pairs separate, so each gamete has one sex chromosome: females only have X, males can provide X or Y.
Therefore, the fertilizing sperm determines the genetic sex of the embryo.
Birds, some fish, some reptiles, and some amphibians have a ZW sex chromosome system, where the male is homogametic (ZZ) and the female is heterogametic (ZW).
So, the female ovum determines the genetic sex of the offspring.
Some insects have only 1 sex chromosome, an X0/XX system (male/female).
Sexual Differentiation
Sexual differentiation is the development of gonadal, neural, and endocrine tissues to determine an animal’s phenotypic sex.
Gonadal Sex Determination
Embryonic gonads retain the potential to differentiate into either testes or ovaries for a short period, so genetic and phenotypic sex can differ.
In placental mammals and marsupials, a delay of up to several weeks occurs before the sex-regulating region of the Y chromosome (SRY region) triggers masculinization.
Without the SRY region (Y chromosome), the gonads develop into ovaries.
Testosterone release from developing testes results in differentiation of male reproductive tract and external genitalia.
The genetic sex and phenotypic sex are usually identical, but differences can arise:
If the animal lacks androgen receptors, the masculinizing effects of testosterone cannot occur.
High concentrations of sex steroids early in development can alter phenotypic sex.
Environmental factors such as temperature or chemical exposure in early development may influence whether testes or ovaries (or mixed) develop.
Temperature-Dependent Sex Determination
The temperature at which eggs are incubated determines the sex in many species of reptiles (no known sex chromosomes).
Studies have identified a thermosensitive period during which incubation temperature can alter gonadal development.
Two patterns exist for the thermosensitive period:
Pattern I animals tend to have a single temperature transition zone:
Subgroup IA: lower temperatures result in males.
Subgroup IB: lower temperatures result in females.
Pattern II animals have two temperature transition zones with males in the intermediate temperature and females at both extremes.
Role of Estrogens and Aromatase
Estrogens are responsible for the differentiation of ovaries.
Testes develop in the absence of estrogens.
The enzyme aromatase converts testosterone (the main androgen) to estradiol.
In temperature-dependent sex determination, aromatase activity starts during the temperature-sensitive period and is controlled by genes.
In some species of reptiles with sex chromosomes, temperature can override their sex chromosomes, resulting in phenotypic males, despite a female genotype.
Environmental Endocrine Disruptors
Glyphosate-based herbicides (Roundup) disrupt aromatase activity and steroidogenesis, potentially leading to endocrine dysfunction, cancer, and infertility.
Atrazine-based herbicides increase aromatase activity, potentially leading to feminization of animals.
Hermaphroditism
Hermaphrodites are animals that produce both male and female gametes.
Simultaneous hermaphroditism: an ovotestis produces both male and female gametes simultaneously; these could self-fertilize but usually exchange gametes instead (mutual insemination, e.g., earthworms).
Sequential hermaphroditism: gonads initially form one type of gamete but then change to the other type.
Protandry: male formed first (e.g., slipper limpet).
Protogyny: female formed first (e.g., sheephead fish).
Vertebrate Reproductive Systems and Gametogenesis
Gametogenesis is the process of developing male and female gametes.
Spermatogenesis and Oogenesis
MITOSIS: Mitotic proliferation produces large numbers of spermatogonia in males and oogonia in females. Two chromosome pairs are shown in each diploid (2n) cell—in each pair, one is the maternal chromosome (red) and one the paternal chromosome (green).
MEIOSIS I: Meiosis I forms haploid gametes (1nchromosomes). Note the random patches of red and green within the chromosomes in the secondary oocytes and spermatocytes due to the exchange of chromosome material between each pair. In females, a secondary oocyte and the first polar body are formed.
MEIOSIS II: Meiosis II generates many spermatids in males that go on to form haploid spermatozoa. A second polar body is extruded when meiosis II is completed, just after fertilization. An ovum is haploid (1n) until fusion with genetic material from the spermatozoa occurs.
Vertebrate Male Reproductive Systems
Fish, reptiles, and birds have internal testes, but most mammals (besides aquatic ones) have external testes.
Testes must remain within a certain temperature range for proper spermatogenesis.
Spermatogenesis is the process of sperm production from male gonads.
The testes are composed of seminiferous tubules with rings of Sertoli cells, which support the development of spermatogonia.
Spermatogonia undergo mitosis to form primary spermatocytes.
Primary spermatocytes then undergo meiosis I to form secondary spermatocytes and then meiosis II to form spermatids.
Spermatids then go through spermiogenesis to become motile spermatozoa.
Meiotic divisions result in equal distribution of cytoplasm.
Spermiogenesis
The transformation of spermatids into spermatozoa is called spermiogenesis, which involves:
Reducing the amount of cytoplasm.
Condensing the chromosomes into the head.
Adding a motile tail.
Spermatozoa are made up of several characteristic features:
A head bearing genetic material in the nucleus.
The acrosome – a cap on the head important in penetrating the ovum during fertilization.
A midpiece with mitochondria.
A tail powered by the mitochondria and formed by microtubule motors.
Capacitation
In most animals, sperm are ready to fertilize an ovum immediately after release.
In mammals, further changes are needed. Capacitation is the process of increasing the fluidity of the acrosomal membrane, resulting in higher permeability to Ca2+ ions, which increases sperm motility and their capacity to bind to an ovum.
Capacitation usually occurs within the female reproductive tract, where sperm must swim rapidly up the oviducts.
Hormonal Control in Males
Hormones of the hypothalamic-pituitary axis are involved in male reproductive function:
Gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary.
LH triggers Leydig cells in the testes to secrete testosterone.
FSH and testosterone trigger Sertoli cells of seminiferous tubules to promote spermatogenesis.
Testosterone and Secondary Sexual Characteristics
Large increases in testosterone at puberty drive the development of secondary sexual characteristics, which occur annually or seasonally in seasonal breeders.
Examples include hair growth, thickening of vocal cords, antlers of deer, territoriality, aggression, song production in birds, and vocalization of frogs and toads.
Vertebrate Female Reproductive Systems
Most vertebrates have paired ovaries and two oviducts.
One ovary and oviduct regress in nearly all birds as well as some fish, lizards, and mammals (like bats).
The oviduct is often just a conduit, but in some animal groups, specialized segments add outer layers or casings to the developing egg (elasmobranchs, birds, reptiles, and insects laying cleidoic eggs).
The magnum adds albumin, the isthmus adds egg membranes, and the shell gland adds the calcium-rich shell and outer pigments.
In viviparous animals, the oviducts fuse to form a specialized region to hold the developing embryo (uterus in mammals).
The uterus of mammals has two layers: a muscular myometrium and a highly vascularized endometrium where the embryo would implant.
Folliculogenesis/Oogenesis
Folluculogenesis/oogenesis is the process of forming follicles/eggs (ova) from the female ovaries.
Oogonia are formed by mitosis from the primordial germ cell.
Before or soon after birth, oogonia enter meiosis I to form a primordial follicle, which contains a primary oocyte, which then enters prophase.
*Note unequal cytoplasm distribution
Hormonal Control of Folliculogenesis/Ovulation
GnRH stimulates the release of FSH from the pituitary, which stimulates follicular growth.
LH stimulates the secretion of androgens; aromatase converts those to estrogens, stimulating cell proliferation.
Mature "Graafian" follicles secrete large amounts of estradiol, which has a positive feedback on LH, stimulating ovulation of the secondary oocyte.
Completion of Meiosis II and Fertilization
Only when the ovulated egg (secondary oocyte) comes into contact with sperm will meiosis II be initiated, forming the ovum.
The sperm then fertilizes the haploid ovum, forming the diploid fertilized ovum (zygote).
After ovulation, most species enter a luteal phase in which the remains of the ruptured follicle form the corpus luteum, which grows in size and begins releasing progesterone.
Progesterone inhibits the hypothalamic-pituitary axis, preventing new folliculogenesis, and quiets smooth muscle contractions.
If the egg is not fertilized, the corpus luteum degenerates, and the animal will return to the next follicular phase.
Declining progesterone leads to the regression of the uterine lining, leading to shedding of the uterine lining in primates (menstrual cycle) or the estrous cycle in non-primates without menstruation.
Spontaneous ovulation occurs in cycles without stimulation.
Induced ovulation (rabbits, hares, ferrets, minks, rats, and camelids) only occurs in response to mating.
Mammalian Ovulation Cycles
Primates (e.g., humans, rhesus monkeys) have a long period of folliculogenesis, followed by a luteal phase of 14 days. Menstruation (menses) occurs between the end of luteolysis and the start of folliculogenesis.
Domestic farm animals (e.g., sheep and pigs) have a short period of folliculogenesis and a longer luteal phase.
Rodents (e.g., rats, mice, hamsters) ovulate every few days until mating. Functional corpora lutea form only after mating and are retained during pregnancy. If the ova are not fertilized, the retention of corpora lutea gives false endocrine signals of pregnancy, known as a pseudopregnancy.
Induced ovulators (e.g., rabbits and cats) experience mating-stimulated ovulation. The corpora lutea remain intact even if fertilization does not occur. The long pseudopregnancy after an infertile mating may be as long as normal pregnancy or terminated by luteolytic mechanisms.
Canine species (e.g., dogs and wolves) usually have two ovulatory cycles per year. After ovulation, a fully functional corpus luteum (or several corpora lutea) forms. In unmated animals or after an infertile mating, the corpus luteum persists in a pseudopregnancy similar in length to a normal pregnancy.
Fertilization and Subsequent Events
When sperm comes into contact with the secondary oocyte, they must first penetrate the outer layer called the corona radiata by digesting the cumulus cells forming the layer.
The acrosomal reaction begins:
The acrosome releases enzymes that digest the outermost layer of the egg.
An extension forms from the sperm head called the acrosomal process.
The acrosomal process binds with species-specific receptors on the egg, allowing the sperm to penetrate and fuse plasma membranes, releasing the sperm nucleus.
Prevention of Polyspermy
Polyspermy is the fertilization of an egg by more than one sperm.
Fast-block: Within 1-2 seconds, the first sperm triggers Na+ ion influx, depolarizing the egg membrane and preventing further sperm binding.
Slow-block: Ca2+ release upon sperm entry stimulates a cascade resulting in cortical granules releasing their contents into the perivitelline space. Protease enzymes released from the granules separate the vitelline and egg membranes. These contents increase the osmolarity of the fluid in the space, drawing water in, lifting the vitelline membrane from the egg membrane, including any other sperm, within ~1 minute. Finally, the vitelline membrane hardens, forming the fertilization membrane, preventing the penetration of any additional sperm.
A fertilization cone then draws the sperm nucleus into the egg, triggering the completion of meiosis II, followed by the fusion of the two gametes to form the zygote.
Fertilization in Teleost Fish
In teleost fish, sperm do not have an acrosome, so instead they enter the oocyte through a funnel-shaped channel called the micropyle.
Polyspermy is prevented because only one sperm can fit.
A fertilization cone and fertilization plug are then formed, blocking the channels within the micropyle and preventing polyspermy.
After fertilization forms the zygote, mitotic division creates the blastocyst, doubling the number of cells with each division, and implanting into the uterus, forming the placenta.
Delays in Reproductive Processes
Sperm storage allows a single insemination to produce fertilized eggs over several days (delayed fertilization), enabling births to be better timed with food availability (birds and bats).
Embryonic diapause is the temporary arrest in embryonic development (some individuals of almost all animal groups) due to environmental conditions (temperature, rainfall, etc.), either before or after implantation.
Lactational quiescence occurs in wallabies and kangaroos where the suckling of an older joey causes delayed development and delayed implantation of the youngest fetus.
Seasonal quiescence also occurs as a result of melatonin secretion due to photoperiod.
Asexual Reproduction
Some animal species can reproduce by asexual reproduction without the fusion of male and female gametes.
Asexual reproduction allows for rapid expansion of populations, but without genetic change, limiting the ability to respond to environmental changes.
Cnidarians (sea anemones, corals, and jellies) and tunicates (sea squirts) can reproduce asexually by fragmentation or budding.
Fragmentation: a new individual can form from a piece of the adult (annelid worms, sea stars, and flatworms).
Budding: a new individual forms directly from the body of the adult via mitosis.
Binary fission: a form where one individual divides into two equally-sized individuals.
In persistent (obligate) parthenogenesis of some arthropods, the unfertilized egg can develop into diploid adults via mitosis that are exact clones of the mother.
Others, like aphids, reproduce by cyclical parthenogenesis, in which there is an alternation between sexual and asexual reproduction: parthenogenic during the summer and sexually forming eggs that enter diapause in winter.
Facultative Parthenogenesis
Facultative parthenogenesis is a method used by usually sexual animals in the absence of or low frequency of males for prolonged periods.
Sharks held in captivity for several years without males suddenly gave birth to young.
Komodo dragons held in captivity separated from males have given birth to viable young.
The mechanisms behind what triggers facultative parthenogenesis are unknown.