Reproduction and development Two modes of reproduction asexual: one parent; offspring are clonal sexual: two parents produce gametes, which fuse to form a zygote “Advantages” and “disadvantages” to each

How does reproduction take place? Asexual- no gametes fission: binary and multiple binary: bacteria and protozoa can be lengthwise or transverse multiple: nucleus divides rapidly before cytoplasm divides seen in some parasitic protozoa budding: new individual arises as out- growth of the old several animal phyla, esp. cnidarians, tunicates

Asexual, continued Gemmulation- formation of cell aggregation surrounded by a capsule (gemmules) Fragmentation- lots of animals can reproduce this way sponges, cnidarians, annelids, tunicates regeneration is part of the process, but some animals can regenerate body parts without actually reproducing

Advantages to asexual reproduction rapid isolated animals can reproduce is successful if animals are well adapted to their environment Some animals can reproduce both ways, depending on circumstances

Modes of sexual reproduction Bisexual- involving two individuals Hermaphroditism Parthenogenesis- one individual

Hermaphroditic (monoecious)- male and female reproductive systems in same individual Usually one individual fertilizes another Some fishes are sequentially hermaphroditic Wrasses start out as females and change to males protogynous (female first) protandrous (male first)

Parthenogenesis (“virgin birth”) Embryo develops from unfertilized egg, or male and female nuclei do not unite Ameiotic parthenogenesis (“asexual”) egg forms by mitotic division occurs in some flatworms, rotifers, crustaceans, insects and others offspring are genetically identical to parent

Meiotic parthenogenesis Ovum is formed by meiosis (i.e., is haploid) sperm may or may not activate egg Various forms of these are described Some fishes: sperm activates egg but is rejected before it fuses with egg nucleus

Haploid egg can begin developing simultaneously chromosomes replaced by duplication Flatworms, rotifers, annelids, mites, insects Bees, wasps, ants Fertilized eggs become diploid females (queens, females) Unfertilized eggs become drones Whiptail lizards become clones of (female) parents

Why parthenogenesis? If males and females cannot be brought together Not viable in mammals (fetuses can develop in mice) Parthenogenesis was achieved recently in mice (with a little genetic manipulation) Kono et al., reported in Nature, 4/21/04

Not the usual type of parthenogenesis Chromosomes form two different female mice were used One was modified so that a certain gene was deleted, allowing a growth factor (IgF2) to be expressed Genetic imprinting; in embryos this gene is expressed on the paternal chromosome Only 2 out of 500 attempts were successful One parthenogenetic mouse reproduced

Sexual vs asexual reproduction Asexual more “energy-efficient” May be advantageous if environment is stable Otherwise diversity provided by sexual reproduction is advantageous

Origin of germ cells (as opposed to somatic Vertebrates Primordial germ cells formed from endoderm Migrate to gonads Develop exclusively into eggs and sperm Invertebrates Distinct germ cells may form, or they may derive later from somatic cells

Sex determination Chromosomal in many animals Sometimes dependent on temperature or other stimuli Alligators: eggs incubated at low temperatures become female; high temperatures, male (no sex chromosomes)

(in humans)

TDF is the product of the SRY gene on the Y chromosome Testes develop much more rapidly than ovaries (7 weeks vs. 15 weeks) TDF initiates a sequence of events that leads to the formation of testes and male external genitalia

Disorders of embryonic sexual development Hermaphroditism- both ovarian and testicular tissue Pseudohermaphroditism congenital adrenal hyperplasia testicular feminization

Sperm vary greatly in size among species Sperm production greatly outnumbers egg production

Oogenesis Oogonia- earliest forms; diploid; divide by mitosis Primary oocytes do not divide equally (polar body) Secondary oocytes are haploid One functional ovum is ultimately formed from a germ cell

In many animals meiosis is not complete before fertilization Birds, most mammals- at ovulation Many invertebrates, fishes, reptiles, amphibians- after fertilization Humans- arrested in prophase I in fetal stage resumes at ovulation is completed only on fertilization Yolk is distinctive: greatly enlarges egg cell

Reproductive patterns- internal, external fertilization Oviparous- egg-laying (invertebrates and some vertebrates) fertilization internal or external Ovoviviparous- eggs retained in body, nourished by yolk (some annelids, arthropods, gastropods, some fishes and reptiles Viviparous- develop in oviduct or uterus, nourished by mother (mostly mammals, some fishes, scorpions)

Reproductive systems Primary organs (gonads) Secondary organs- assist with formation and delivery of gametes may support embryo

Invertebrates may expel gametes, som system is very simple insects can be quite complex

Reproductive cycles estrus- most mammals brief receptivity to male during cycle menstrual cycle sexual activity can occur throughout cycle uterine lining is shed

(human) Estrogen surge causes release of GnRH. This causes release of FSH and LH

Pregnancy and birth Fertilization usually occurs in uterine tube Blastocyst is formed by the time it reaches the uterus

“Extraembryonic membranes” Start forming after implantation Yolk sac- transport of nutrients, red blood cell formation. Role reduced> 6 weeks Amnion- encloses amniotic cavity. Fluid cushions developing embryo/fetus Allantois- forms urinary bladder; umbilical cord Chorion- blood vessels help nourish embryo; develops into placenta. Secretes hCG, which stimulates corpus luteum to secrete estrogen and progesterone

Placenta Umbilical arteries and veins provide fetal circulation Maternal circulation does not actually mix with fetal blood Gas and nutrient exchange takes place here Secretes estrogen and progesterone to maintain endometrium (corpus luteum does that up to 3rd month)

Four weeks

Labor and childbirth Labor oxytocin (hypothalamus) prostaglandins Fetal adrenal gland produces cortisol and an estrogen presursor; makes uterus more sensitive to oxytocin and prostaglandins CRH secretion by placenta triggers fetal adrenal gland activity

Single or multiple births? Multiparous- several eggs develop at once Armadillos always give birth to four offspring, all the same sex Humans tend to be uniparous twinning is monozygotic or dizygotic (“identical” or “fraternal”) Monozygotic twinning is uniform Dizygotic twinning seems to vary with ethnicity and/or geography

Monozygotic twinning One fertilized zygote splits and forms two embryos Depending on timing of split, twins may develop separate placentas (2-cell stage); one placenta and two amnions (complete split of cell mass) or share placenta and amnion (later in development; conjoined twinning is a risk here)