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.
BROAD PRINCIPLE
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.
In asexual reproduction the progeny is
genetically_____________. In most cases of
sexual reproduction the progeny is genetically
_________________
A)
B)
C)
D)
variable, identical
Identical, variable
Variable, also variable
Identical, also identical
The Many Guises of Asexual Reproduction
1) Budding and fission (a new individual arises from
outgrowths of existing ones -sponges, cnidarians
or by division of an individual).
Anemone during fission
2) Fragmentation and regeneration (a new
individual arises from broken pieces of
another animal’s body -sponges, cnidarians,
planaria).
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
“The hypothalamic-pituitary axis”
Luteinizing
hormone
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
A) Inhibits the release
of GnRH and hence
ovulation and the
ovarian cycle.
B) Prevents the corpus
luteum from
degenerating.
C) 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 _______________.
A)
B)
C)
D)
E)
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: GnRH production 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 (Apis mellifera, 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 extrapair copulations.
Frequency of various mating systems in birds and mammals
Mating System
Birds
mammals
Monogamy
92%
sometimes
(carnivora, voles)
Polygyny (1 male
many females)
2%
often
Promiscuity
6%
often
Polyandry (1
female many
males)
≈ 0.5%
almost never
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
Sage grouse male displaying
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
Tibetan fraternal polyandry
Saddle-backed Tamarin
(Saguinus fuscicollis)
Human societies show all forms of mating systems
Polygyny (Islam, classical China, early Hebrew, early Mormonism, 2/3 of all
societies accept polygyny)
Monogamy (Most humans live in monogamous pairs, Ancient Greece and Rome,
mainstream Christian societies. Only ≈ 1/3 of all societies sanctify monogamy).
Polyandry (Tibetan fraternal polyandry. Others?)
Promiscuity (all over the place)
Religions often sanctify the moral appropriateness of a given mating system
Bigamy is having one wife (husband?) too many.
Monogamy is the same.
Oscar Wilde
≈ 0.5% of all bird species are polyandrous. These species are socially as
well as reproductive polyandrous. Polyandrous birds also show “sex role
reversal”. Sex role reversal means male only care for the young, the
female is larger and more colorful, and parental care hormone profiles
are reversed. There are two forms of polyandry in birds:
Northern Jacana (Jacana spinosa) Spotted sandpiper
(Acitis macularia)
Simultaneous polyandry:
Red-necked phalarope
Each female holds a large
(Lobipes lobatus)
territory that contains
the smaller nesting
Sequential polyandry: A female mates
territories of two or more
with the male, lays eggs and then
males who incubate eggs
leaves him to repeat the sequence with
and tend the young.
another male.
In Jacanas the female defends territories, shows
territoriality, but does not help in chick raising. This role is
played only by the male. In Jacanas you expect prolactin to
be ____________ in males and ________ in females,
whereas testosterone is ____________ in males and
________ in females.
A) high, low, low, high
B) Low, high, high, low
C) High, high, low, high
D) Low, low, low, high
E) High, high, high, low
Testis size and mating systems
Monogamous?
Polygynous?
1.50
Ln(testis mass)
1.00
Nope…Polyandrous!
0.50
0.00
-0.50
Monogamous?
Polygynous?
-1.00
-1.50
-2.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
Nope…lek species in
which females mate with
one male but males mate
with several females.
Ln(body mass)
In birds (as in mammals) relative testis size is the largest in animals in which there
is strong sperm competition, such as in polyandrous species and species with lots of
extra pair parentage.
Homosexuality (sexual
behavior between
members of the same sex)
is relatively common
among vertebrates…
≈ 1/3 of all Laysan albatross nesting pairs are
female/female. They raise young successfully.
Can Animals Be Gay? (article posted in my web site)
Review Questions posted