Sex and Reproduction (not the same thing!!)
Animals reproduce by both sexual and asexual means. Asexual reproduction is the creation/production of new individuals in which all the genes come from a single parent. In most (but not all) cases asexual reproduction involves only mitotic cell divisions. Sexual reproduction is the creation of new individuals by the fusion of haploid gametes to form a zygote (fertilized egg). The male gamete is called sperm and is usually small. and motile. The female gamete is larger, immobile, and called an unfertilized egg (ovum).Sexual reproduction involves meiosis and hence recombination (gene shuffling). It generates variation. BROAD PRINCIPLE
In asexual reproduction the progeny is genetically_____________. In most cases of sexual reproduction the progeny is genetically _________________ • variable, identical • Identical, variable • Variable, also variable • Identical, also identical
The Many Guises of Asexual Reproduction 1) Buddingand fission (a new individual arises from outgrowths of existing ones -sponges, cnidarians or by division of an individual). 2) Fragmentation and regeneration (a new individual arises from broken pieces of another animal’s body -sponges, cnidarians, planaria). Anemone during fission Hydra budding Regeneration in Planaria
3) Parthenogenesis (a new individual arises from the unfertilized eggs of another one ). Several species of Daphnia alternate between parthenogenesis and sexual reproduction
Cyclical parthenogenesis can also be found in aphids baby aphid Note that asexual reproduction produces many genetically identical individuals. In contrast, sexual reproduction produces individuals that are genetically variable.
General principle: When the going gets tough, the tough have sex….. And produce a variable progeny. BROAD PRINCIPLE
A very curious form of asexual reproduction occurs in several whiptail lizards (Cnemidophorus), which have no males. The females alternate between male and female behaviors. Females produce haploid (N) ova by meiosis. The ova undergo a chromosomal duplication (2N) so that the lizards are diploid. The lizards still require mating to reproduce successfully… But they do not require males. Other species “parasitize” males from closely related species (some fishes).
TO REMEMBER -Asexual reproduction is the creation/production of new individuals in which all the genes come from a single parent. -Sexual reproduction is the creation of new individuals by the fusion of haploid gametes to form a zygote (fertilized egg). IT ALWAYS INVOLVES MEIOSIS AND RECOMBINATION. -Asexual reproduction leads to low genetic variation, sexual reproduction leads to high genetic variation in progenies. -The male gamete is called sperm and is usually small. and motile. The female gamete is larger, immobile, and called an unfertilized egg (ovum). -Many animals (Daphnia, aphids) alternate between asexual and sexual reproduction. This is called cyclical parthenogenesis.
Nature’s Rainbow Many animal species have separate sexes (dioeceous). Many others have both sexes in a single individual. An individual with both sexes is called a hermaphrodite. Eros (From a combination of Hermes And Aphrodite Hermpahrodite examples can be found in annelids, flatworms (like this fluke), mollusks,…, etc.
Hermaphroditic garden snails mating Love dart
Some fishes show sequential hermaphroditism. Some are female when they are small and shift to male when large (like the bluehead wrasse shown in the picture). Others do the opposite… Male Female
Males and females have different anatomies, and often vastly different reproductive strategies. Each produces gametes, but these gametes are very different (the production of different gametes is called anisogamy). Some theoreticians have argued that the difference in cost and investment of the two sexes explains the contrasting traits of males and females. Aniso = different Gamy = seed
ureter Secretion of seminal fluid Secretion of prostatic fluid Buffering of acidic urine
Males produce sperm (small, motile, numerous) Leydig cells make testosterone Nurture spermatogonia
Gonadotropin releasing hormone Follicle stimulating hormone Luteinizing hormone “The hypothalamic-pituitary axis”
TO REMEMBER -From diagram:ureter, urinary bladder, testis, epididymis, vas deferens, ejaculatory gland, prostate, urethra. -Seminiferous tubes, what do Leydig and Sertoli cells do?. -The hypothalamus secreted godadotropin releasing hormone (GnRH) into the anterior pituitar. -The anterior pituitary releases 1) Follicle stimulatng hormone (FSH) and 2) Luteinizing hormone (LH). -FSH stimulates Sertoli cells, LH stimulates primary and secondary sex characteristics (by stimulating the Leydig cells to make testosterone). Both stimulate spermatogenesis.
To remember: Ovary, oviduct, uterus (wall and endometrium), cervix, vagina clitoris
The Ovarian cycle 1) In humans the ovarian cycle begins when the hypothalamus releases GnRH (gonadotropin releasing hormone), which 2) stimulates the pituitary to secrete small amounts of LH (luteinizing) and FSH (follicle-stimulating hormone). 3) FSH aided by LH stimulates follicle growth, and 4) the cells of the growing follicle start to make estrogen. During most of the follicular phase there is a slow increase in estrogen levels. When the secretion of estrogen by the follicle begin to rise steeply 5) the levels of LH shoot up 6). The maturing follicle develops a large cavity and forms a bulge near the surface of the ovary. Ovulation takes place about 1 day after the LH surge, and marks the end of the follicular phase. Following ovulation LH stimulates the transformation of the follicle into a glandular structure called the corpus luteum, which secretes progesterone and estrogen.These hormones in combination inhibit (in a negative feedback) the secretion of GnRH. As the corpus luteum disintegrates the levels of these ovarian hormones drop and the hypothalamus begins to secrete GnRH again.
To Remember -The ovarian cycle can be divided into two phases: The follicular phase and the luteal phase. -During the early follicular phase GnRH secretes LH and FSH which stimulate the growth of the follicle. -The follicle produces estrogen, which leads to a spike in LH secretion which precedes ovulation. -Ovulation marks the transition between the follicular and the luteal phases. -The corpus luteum produces progesterone and estrogen, if there is no fertilization, the corpus luteum dissappears (and progesterone and estrogen levels go down). -If the egg is fertilized, the corpus luteum persists. Keeps on producing estrogen and progesterone, which inhibit the secretion of GnRH by the hypothalamus) and hence prevent further ovulation.
The menstrual (uterine) cycle The hormones secreted by the ovaries (estrogen and progesterone) have a major effect on the uterus. Estrogen secreted in growing follicles signals the endometrium to thicken . The latter part of the follicular phase is coordinated and overlaps with the proliferative phase of the menstrual cycle. Even before ovulation, the uterus is is getting ready for the embryo. After ovulation (9), estrogen and progesterone stimulate continued development and maintenance of the endometrium (lots of arteries and glands). The luteal phase in the ovary is coordinated with the secretory phase in the uterus. 10) The drop in levels of ovarian hormones when the corpus luteus disintegrates, causes spasms in the uterine lining that deprive it of blood. The upper two thirds of the endometrium slough off resulting in menstruation (this is the menstrual flow phase).
To Remember -The ovarian cycle is accompanied by a uterine cycle of changes in the uterus. -The uterine cycle in humans has 3 phases: the menstrual flow phase, the proliferative phase, and the secretory phase. -During the proliferative phase the endometrium thickens in preparation for the implantation of the egg. -The transition from the proliferative to the secretory phases coincides with the transition from the luteal to the follicular phase. -If there is no implantation, the menstrual phase coincides with the early phases of the luteal phase.
To Remember -The details of both the ovarian and menstrual cycles… -If there is implantation the corpus luteum (and later the placenta) continues to produce progesterone. Progesterone maintains pregnancy by blocking the sloughing off of the endometrium (which nurtures the implanted fertilized egg).
How do contraceptives work? • “The pill” usually contains a mixture of progesterone and estrogen. The pill • Inhibits the release of GnRH and hence ovulation and the ovarian cycle. • Prevents the corpus luteum from degenerating. • Favors the sloughing off of the uterine lining.
RU486 (mifepristone) blocks progesterone by binding to its receptor. If used within 7 weeks of implantation, RU486 _______________. prevents maintenance of pregnancy. allows the sloughing off of the endometrium prevents the corpus luteum from degenerating Prevents ovulation A and B
How do contraceptives work? The pill: GnRHproduction is blocked by progesterone and estrogen in combination (the pill mimics the luteal phase). Without GnRH there is no ovulation. MAP (progestin): Prevents 1) ovulation, 2) fertilization or 3) implantation (78%). Within 3 days of intercourse. Not clear how it works. RU486 (mifepristone) blocks progesterone. If used within 7 weeks of implantation prevents maintenance of pregnancy. It is taken with prostaglandin which induces uterine contractions.
Menstrual and Estrous Cycles Humans and a few other primates (great apes) have menstrual cycles, which last a month (or “Menses”). Most other mammals have estrous cycles. In both menstrual and oestrous cycles, ovulation starts at a time when the endometrium is thickening. In menstrual cycles the endometrium is shed through the cervix and vagina in a bleeding called menstruation. In estrous cycles, the endomethrium is (mostly) reabsorbed by the uterus and there is no extensive bleeding (there can be some bleeding….). During estrous cycles the behavioral changes are more pronounced. Human females may be receptive to sexual activity through the cycle, whereas most female mammals will only copulate during the period surrounding ovulation. This period of sexual activity (heat or estrus, from L. oestrus = frenzy/passion) is the only time that the condition of the vagina permit mating. Estrous is called “heat” because temperature increases slightly during ovulation. The length of estrous cycles varies widely among species.
Mammals can be monoestrous (cycle once a year), diestrous (twice a year, like dogs), polyestrous and cycle year round (sows, cows) or be seasonally polyestrous and cycle only during certain seasons (ewe and doe). The distinction is not hard and fast, mares are polyestrous except in late fall and early winter, with some exceptions). Anestrum (little or no ovarian activity) Proestrum (follicle is enlarging proliferation of endometrium) Diestrum (uterus is soft and dilated, There may be a little blood) Estrum (female is receptive, uterus is contracted, cervix is dilated and mucus in the vagina is copious)
TO REMEMBER -Oestrus cycles are divided into roughly 4 phases: proestrum (follicle growing, endometrium proliferates), oestrum (receptivity, uterus contracted, cervix dilated), diestrum (uterus dilated), and anestrum (no ovarian activity). -Animals can be monoestrous, diestrous, and polyestrous (seasonal and aseasonal).
Reproductive Behavior Reproductive behavior includes seeking mates, choosing among potential mates, competing for mates, and in some cases caring for young. We will only discuss one topis of this rich theme: Mating Systems
The mating relationship between males and females varies a great deal. We can subdivide mating systems into 3 categories (sometimes the individual within a species fall into more than one of these categories). 1) Promiscuous (no pair bonds or strong relationships) 2) Monogamous (one male+one female) 3) Polygamous (an individual of one sex mates with several of the other) We can divide this category into -Polygyny (one male several females) -Polyandry (one female several males) Gynos=female Andros=male Gamy=mating
Monogamy Monogamy is found in honeybees (Apismellifera, is it?), it is very (very!!) rare in mammals, but it is very common in birds (90% of all species are socially monogamous). Why be monogamous if you are a male? Two Hypothesis 1) Mate guarding (some shrimp spend a week with the female, females are scarce and widely distributed) 2) Mate assistance (males remain with female to help her rear offspring)
In mammals in the species that are monogamous, such as Prairie Voles, Djungarian Hamsters, California mouse, and Arctic foxes, the reproductive success of single females is very low. This idea supports the mate assistance hypothesis but much more research is needed. Peromyscus californicus Lagopus alopex Phodopus campbelli
The sex that “practices” parental care has elevated levels of the hormone prolactin. BROAD PRINCIPLE
Most (but not all) birds are socially monogamous. We use the words “social monogamy” to emphasize that some animals may seem monogamous, but when you use genetic analyses, you find that the progeny of a female may comes from many fathers (extra pair parentage). Indeed, in birds 10-25% of young are the result of extra-pair copulations.
Frequency of various mating systems in birds and mammals Why do birds exhibit a greater propensity to monogamy (with high levels of paternal investment) compared to mammals? Maybe internal fertilization in mammals gives males a prior opportunity to desert. In birds, the egg is detached from the female earlier, so the incentive to provide care is greater for male. Who knows…
TO REMEMBER -Meaning of the terms: promiscuity,monogamy, polygamy, polygyny, polyandry. -Which one of these is most common between birds and mammals.
Some interesting mating systems: Leks: aggregations of males,each defending a tiny “territory” and seeking to attract a mate, and each displaying “passionately” to do so. A lek of ruffs (Philomachus pugnax) in the arctic Sage grouse (Centrocercys urophasianus) lek (locally sharp tailed grouse also display in leks) Some insects, such as the Medfly (Ceratitis caitata) Mediterranean fruit flies
An important characteristic of leks is that one (or a few) males monopolize the copulations. There is a lot of variance in male success. Each successful male copulates with many females, but each female copulates with only one male.
Polygyny is very common among mammals. It occurs whenever females are gregarious (elk, antelope), it can take the form of seasonal (elephant seals, elk) or permanent harems (some carnivores, baboons, zebra). Polyandry is very rare Saddle-backed Tamarin (Saguinus fuscicollis) Tibetan fraternal polyandry