BIOL 371: Theme 5 - Animal Reproduction and Development

animal reproduction

new individuals are produced by existing individuals, generally involves dominant diploid stage producing haploid gametes which fuse to produce new diploid zygote (but not always), zygote must undergo growth and differentiation, cells that give rise to gametes isolated early in development

asexual reproduction

genes from one parent - methods include fission, budding, fragmentation, parthenogenesis

based upon mitosis

pros - don’t need males, rapid reproduction, saves energy

cons - lack of genetic variability

fission

genetically-controlled, two identical individuals result

budding

smaller offspring arises from parent

budding

smaller offspring arises from parent

fragmentation

each fragment gives rise to a new organism

parthenogenesis

new individual develops from unfertilized egg

sexual reproduction

genes from two parents

eggs (oogenesis) and sperm (spermatogenesis) - the gametes (produced by meiosis)

always involves fertilization and development

can be dioecious or monoecious

pros - generates genetic variation

cons - fertilization necessary (complicated), development must proceed from zygote (single cells), cost of eggs and sperm, males don’t produce offspring

dioecious (gonochoristic)

two separate sexes - male or female

monoecious (hermaphrodite)

can be simultaneous or sequential, an individual has male and female reproductive organs (simultaneously or at different life stages)

gametes

cells that give rise to these (germ cells) are developmentally isolated from somatic cells

not affected by development and differentiation of somatic cells, experiences of organism do not affect genome of sex cells

gametogenesis

takes place in primary reproductive organ or gonad, consists of mitosis, meiosis and cell differentiation to produce gametes (haploid sex cells which must combine to give rise to zygote - new diploid individual)

*gametes are the only haploid stage of an animals’s life cycle

sperm

produced by spermatogenesis, highly modified and simplified (mitochondria provide ATP for flagellum, head consists of acrosome, nucleus, centrioles, few or no stored nutrients)

male strategy is to produce abundant, small, motile, cheap gametes

ovum

produced by oogenesis, contains large amounts of cytoplasm, organelles, stored nutrients (yolk), contained within vitelline membrane (zona pelucida) - carries receptors for sperm, may be wrapped within jelly layer, hard shell

female strategy is to produce few large, non-motile, well-provisioned gametes, reciprocal of male strategy

male reproductive system

testes produce sperm, sperm ducts carry sperm to genital opening, eg. vas deferens in mammals

external reproductive structures depend upon whether fertilization is external or internal

female reproductive system

ovaries produce eggs, oviducts carry eggs to genital opening

external reproductive structures depend upon whether fertilization is external or internal

hermaphroditism

adult organism possesses functional reproductive systems of both sexes, either simultaneously or sequentially, common in some taxa, eg. annelids, gastropods

fertilization

bringing sperm and egg together, the right sperm (one) for the egg (don’t want the wrong species, could lead to a polyploid zygote or hybridization)

early embryo development

protection, events - cleavage, blastula, gastrula (tissue layers), pattern formation (basic body plan - anterior, posterior, right, left, etc), organogenesis (formation of organ rudiments)

external fertilization

broadcast spawning, releasing sperm into water and hope it reaches eggs

only in aquatic animals, gametes generally small (egg is still larger), frequently high egg mortality, number of eggs released varies greatly among species, coordinated gamete release may be due to stimulus through courtship movements, calls, or pheromone release, or in response to seasonal environmental cues (eg. full moon)

internal fertilization

found universally in terrestrial and in some aquatic animals, large differences in gamete size, number of eggs relatively small

copulation (sexual intercourse) - generally requires copulatory organ

spermatophore - alternate to copulation - package of sperm and mucus introduced into female’s reproductive tract

allows for different ways of protecting the developing embryo, eg. shelled egg, female retains egg in reproductive tract, can exchange gas and often nutrients

challenges for fertilization

making sure that the right sperm encounters the egg and that only one sperm fertilizes it

wrong sperm —> possibility of interspecific sperm egg encounters (external fertilization), or possibility of mating with an individual of the wrong species (internal fertilization)

requires reproductive isolating mechanisms (prezygotic and postzygotic barriers)

preventing polyspermy

2 parts - acrosomal reaction (fast block) then cortical reaction (slow block)

acrosomal reaction

1st, fast block, sperm cell membrane fuses with egg cell membrane, initiates wave of depolarization - subsequent sperm can’t fuse with egg

cortical reaction

2nd, slow block, cortical granules fuse with egg membrane, vitelline coat becomes impenetrable to sperm - external and internal fertilization

prezygotic barriers

eggs and sperm never encounter one another, or if they do, fertilization does not take place

eg. habitat isolation, temporal isolation, behavioural isolation, mechanical isolation, gametic isolation

also prevents gametes from being waste, increases good reproductive opportunity

postzygotic barriers

fertilization can take place, but hybrids unlikely to reproduce in turn (prevents gene flow)

eg, reduced hybrid viability, reduced hybrid fertility (sterile), hybrid breakdown

jelly layer

on aquatic eggs, protection of the embryo, unappetizing to avoid predation

amniotic egg

a shared derived character, variations found in all amniote vertebrates, extraembryonic membranes carry out life-support functions for embryo, shell protects internal environment, requires internal fertilization

viviparity

exchange of gasses, nutrients and wastes between mother and developing embryo, requires development of some form of placenta for exchange - in vertebrates, derived from extraembryonic membranes, found in mammals and some reptiles, some fish, but not birds

ovoviviparity

retention of the fertilized egg within the mother’s body, with varying degrees of exchange of gasses, nutrients or wastes between mother and developing embryo

frequently seen as intermediate step between oviparity and viviparity (actually a continuum)

ontogeny

early development, cleavage —> gastrulation —> morphogenesis —> organogenesis —> growth

mitosis, gene activation/expression, differentiation

zygote must give rise to a complex multicellular body

vegetal pole

of the ovum, primarily yolk - nutrient supply, slow cleavage, producing large cells

animal pole

of the ovum, where egg nucleus is located, relatively rapid cleavage, producing small cells

developmental information

stored in nucleus and cytoplasm (maternal cytoplasmic determinants - mRNA and proteins from mother’s genome, molecules which help direct development and differentiation in embryo)

maternal cytoplasmic determinants

mRNA and proteins from mother’s genome, molecules which help direct development and differentiation in embryo

cleavage

mitosis, zygote immediately begins undergoing cleavage, mitotic division with no increase in mass, using egg’s resources, may be spiral or radial

process results in morula (solid ball of cells) —> gives rise to blastula (hollow ball of cells - blastomeres, has animal pole and vegetal pole)

cytoplasmic determinants

distributed unevenly through egg cytoplasm, cleavage divides them up - different blastomeres have different combinations and amounts of them

blastomeres undergo differentiation - become different types of cells depending upon original position (spatial distribution of cytoplasmic determinants determines direction of cleavage spiral in snails)

syncytium

fertilized egg undergoes repeated division without cytokinesis

blastulation

cytokinesis —> hollow ball of cells/blastomeres

gastrulation

segmentation, formation of ecto/meso/endoderm, gut, mouth and anus, initial establishment of body axes

process by which blastula changes into embryo - composed of three germ layers

accomplished by movement and differentiation of cells (morphogenesis)

actual events of gastrulation vary greatly according to nature of blastula

endoderm

lining of gut and associated organs

mesoderm

muscle, other internal organs, connective tissue and skeleton

ectoderm

outer surface, skin, nervous system

drosophila

developmental model organism, one to one correspondence between segmented larva and adult - developmental regions landmarked, developmental changes easily followed

bicoid

gene controlling formation of anterior segments in drosophila embryo

maternal-effect inheritance - expressed in female fly, contributed to egg before fertilization

maternal cytoplasmic determinant - maternal mRNA from gene not evenly distributed through egg

expressed strongly at anterior end of egg, protein produced after fertilization and diffuses down length to establish concentration gradient

cytokinesis in blastulation isolates differing bicoid protein concentrations within individual cells - acts as a transcription factor, switching genes off/on depending upon role in producing anterior structure

hox genes

homeotic genes, produce transcription factors which produce segment-specific structures along length of embryo

drosophila hox genes homologous with hox genes in other taxa - genetics of pattern formation appear to be strongly conserved among animalia, homologous genes regulating similar developmental processes or associated with similar structures, ie. aniridia/eyeless

important cellular processes in morphogenesis

mitosis, cell movement, selective cell adhesion, apoptosis, induction, determination, differentiation

apoptosis

programmed cell death, eg. hands and feet are webbed early in development, tissues die after

induction

one group of cells affects developmental path taken by another group of cells through signal molecules or direct contact

determination

switching of most of the genes of the cell, only allowing certain developmental pathways to be followedd

differentation

after determination, cells follow specific developmental pathways

frog gastrulation

the amount of yolk affects the form of the blastula and thus the process of gastrulation

organogenesis

process of cell differentiation and division from initial three germ layers

produces tissues, rudiments or organs and organ systems - uses same cellular mechanisms as gastrulation

cell type specialization produced by interaction of cascades of cell-cell signals and cytoplasmic determinants, what is regulated is expression of the cells’ genome, each cell type has a unique constellation of genes turned on and off - determination - cells begin producing products/behaviours of cell type implicit in determination

neurulation

an example of organogenesis

process of cell differentiation, growth, apoptosis, producing dorsal hollow nerve tube in chordates - induced by chemical signals from notochord (induction)

main effect of signals is to cause reorientation of neural plate cell cytoskeletal elements, causing folding

direct development

new individual is similar to adult (only smaller in size), no dramatic change in morphology

indirect development

new individual different from adult, undergoes metamorphosis