Organismal Biology

Key Concepts

Chapter  11 - Animals -
Reproduction & Development

Reproduction

The basic idea of reproduction is one of immortality, but even for organisms that do not age (and there actually are many of those), individuals are always at risk of death.  Reproduction allows individuals to produce copies of themselves (or of parts of themselves);  one individual may die, but copies live on.

There tends to be a reverse correlation between the odds of an individual surviving to reproductive age and the number of offspring typically produced:  the lower the odds, the greater the number of offspring.

This brings us back to the concept of asexual reproduction, where offspring are genetic copies of the original:  from an immortality aspect, this is true reproduction, making offspring with all of the potential of the original.  There is a negative side to copies, though:  they all have the same vulnerabilities, so a threat to one is potentially a threat to all.  This lowers the odds of survival in any environment where conditions could change against the organism (which is pretty much any environment).

So asexual reproducers produce huge numbers of offspring -  this spreads them out, so often at least one of them is beyond dangerous changes.  Also, since making genetic copies is a process where mistakes are common, the more offspring, the more the chance that many will not be identical copies.  This produces a tiny chance that individuals will be produced that will be able to survive changes that kill the rest.  Consider a state-run lottery game - the odds of an individual getting a winning ticket are incredibly small, but there are so many players that it's more unusual for there to not be a winner for a particular game.

For sexual reproduction, genetic recombination happens in the offspring - the two sets of chromosomes are reduced to one set (with the combinations proportional to the number of pairs), and then sets are recombined in the offspring.  This can be done with single parents (where new combinations can occur, with some chance of producing genetic copies in offspring, especially if the chromosome number is low) or pairs, and the gender roles of male and female are extremely common in animals.

In animals, sperm are produced in testes - starting cells undergo meiosis, a two-division process that produces four haploid cells which are generally transformed into functional sperm.  Animal sperm are mobile, usually swimming with a flagellum but sometimes crawling with pseudopods.  Ova (egg cells) are produced in ovaries, where meiosis is still a 2-division process making 4 cells, but only one cell (the one retaining all of the stored food for the offspring) becomes a functional ovum.  The other three cells, the polar bodies, are tiny cells for discarding of extra chromosome sets.  Typically, the divisions occur when no other complex functions are happening:  not during food storage or sperm penetration.  This means that, in many species, what the sperm nucleus enters during fertilization is not an egg cell, since it has not completed meiosis yet.

Animal systems also have accessory organs, which can be for storage of cells, production of egg-food / yolk materials, packaging of gametes and offspring, and transfer of sperm.  Primary features are those that absolutely must work for reproduction to be successful;  secondary features are those that are important to successful reproduction but not absolutely necessary.

Animals are often dioecious, with separate male and female individuals (even in species where individuals can change their gender, so long as they're only one functional gender at a time).  Gender can be "locked" into a chromosome pattern, such as an "extra" chromosome (like the Y chromosome in mammals or the V chromosome in birds) or a single-double difference, as is found in some insects, or gender may be more fluid, expressed in response to the enviornment (like the temperature-related process for crocodilian eggs).  And as mentioned earlier, how these mechanistic distinctions play out in behavior is an amazingly broad range of possible roles.

Animals can be monoecious, where individuals have functional male and female parts at the same time.  Some may be capable of self-fertilization, and some are not, but every encounter between adults can potentially produce offspring from each partner.

Alternation of generations happens in some animal groups, usually under two basic circumstances:  a generally non-mobile form (asexual) alternating with a sexual mobile form (such as is found in corals), or parasites maximizing the potential of both approaches, often doing each stage in different hosts.

Development

With sexual reproduction, each new generation begins with fertilization, a haploid sperm nucleus fusing with a haploid ovum nucleus, producing a diploid zygote.  Usually, many sperm are trying to open a passage for their nuclei in the ovum membrane, involving an interaction between sperm enzymes and the membrane molecules.  It would be bad for more than one sperm nucleus to fuse with the ovum nucleus, producing polyploidy, too many code copies for every protein;  the egg cell can change its membrane chemistry in an instant once one nucleus gets through.  If, however, this doesn't completely work, there are different responses in different species.  In some, all but one sperm nucleus is expelled or broken down.  In some, so many offspring are produced that the polyploidy losses are just losses.

As active mobile cells, sperm have aerobic respiration chambers, mitochondria, producing energy molecules for the trip.  These have their own small, prokaryote-like genomes, which might not be compatible with the mitochodria of the ovum, and which are excluded from entering.  Mitochodrial genes do not recombine.  Individuals carry mitochondrial copies from their mothers.  Some rare conditions are a result of the exclusion not working.

For many aquatic (water-living) species, ova and sperm are released into the environment, and fertilization is external.  In full-time land animals, and aquatic animals where the fertilization site is a bad site for eggs or offspring, fertilization in internal, and sperm must be introduced into the body of the female.

Once the zygote divides (by mitosis, where each new cell gets a complete genome copy), it is an embryo.  A word commonly used for early divisions is cleavage.  The function of an early embryo is to make a lot of cells;  they are absorbing nutrients from the stored food and growing a bit between divisions, but typically they are getting smaller and smaller.

There are two main "trunks" of the upper animal family tree.  In the protostomes, the divisions produce cells of unequal size.  The smaller cells are pushed to the outside, where they form a spiral pattern, and this is called spiral cleavage.  In the deuterostomes, divisions produce cells of equal size.  Looking down at the first few divisions, the first split is a diameter, then a perpendicular diameter, then more radii, giving this pattern the name radial cleavage.

Protostomes follow what is called a determinant pattern - very early on, cells are altered epigenetically so that their fates are set.  So a cell in the 32-cell embryo will go on to be the front of the digestive system, one will be a specific part of the musculature, etc.  Deuterostomes have a indeterminant pattern - the cells do not specialize until much later, and often retain some ability to change specializations even later.

The following is a very simplified version of a process that can vary a lot in different groups:

The ball of dividing cells at some point hollows out and becomes a single layer of cells around a space (sometimes the food is in that space).  This structure is called a blastula.  Cells in different places are accessing different genes and shifting in functions.  One spot on the outer layer becomes a "dent," gets deeper and cells migrate inward; eventually, the dent becomes an inner layer of cells, parallel to the now-outer layer, the endoderm.  The layer still on the outside is the ectoderm.  Eventually, in many animals a middle layer, a mesoderm, will form - in protostomes, where the other layers meet;  in deuterostomes, as outpocketings of the endoderm.  The opening from the outside in is the blastopore.

The ectoderm eventually is used to produce, not surprisingly, the outer structures of the animal (skin, exoskeleton, etc.), but also the nervous system.  The endoderm becomes the digestive system and related structures (in protostomes, the blastopore becomes the mouth;  in deuterostomes, the anus or umbilical opening).  The mesoderm becomes many of the inner systems:  muscle, circulation, excretion, endoskeleton, etc.  In species with body cavities, a coelom originates with a mesodermal lining.

Using different genes, the cells differentiate, eventually becoming physically as well as chemically different from each other.

Organization Levels

The complexity of organisms moves from protoplasmic, organization within cells, to cellular, which may include simple cell types and extracellular elements, to tissues, classes of cells with particular broad functional or stuctural features, to organs and structures, collections of tissues in a particular place to do one or a few particular functions, to organ systems, collections of organs and structures to do basic life functions.  The systems work together inside an individual organism.

Tissue Types

In animals, there are four or five basic tissue types, with a few important subtypes.  The study of animal tissues, usually of stained sections for transmission microscopes, is called histology.

Epithelium or epithelial tissue is one-to-a-few layers of cells sitting on a basement membrane, facing some sort of space (not always a recognizable space on a section).  The cells can be flat, squarish, or tall.  The layers may be protective or separatory, materials may be absorbed or secreted across them, and secretory epithelium is a common component of glands.

Connective tissue is made up of cells that exists in a noncellular matrix that may or may not be a product of the cells.  Much of endoskeleton structures are made of connective tissue - bone, cartilage, ligaments, tendons.  Fibrous connective tissue often holds parts together internally. Blood is usually considered a connective tissue.  Fat is also considered a connective tissue.

Muscle, with cells that are capable of contracting, has a few particular subtypes.  The weakest, smooth muscle, is used for repetitive movement not requiring a lot of power - it's often found on internal tubes that move materials, such as digestive tubes.  The other three subtypes have much more power, which can be seen in their microscopic striated pattern - stripes that come from the highly organized pattern of interacting muscle proteins.  Cardiac muscle has a rhythmic contraction ability;  the cells are short, branched (good for wrapping around pump spaces) and "zipped" together.  Skeletal muscle has long cells that begin as separate cells but fuse together into long multinucleated cells during development.  Fibrillar muscle is like skeletal muscle but can contract several times with a single nerve command - it's found in many insect flight systems.

Nervous tissue is capable of generating and passing along an electrochemical signal, a nerve impulse.  Sense receptors convert environmental input to impulses that run along sensory neurons.  Interneurons relay, process, and store information.  Motor neurons activate muscles and glands.  Most neurons are separated by tiny gaps, synapses, that must be "bridged" by a release of neurotransmitter molecules.

Reproductive tissue is not always considered a separate tissue, but if it is, it is the meiotic cells and gametes in reproductive systems.

Basic Layouts

When differentiation begins, a special class of genes activate that are very similar across most animal groups.  These Hox genes are basic layout determiners, setting things like front and back ends, limb placement, segment designation, and other items of general architecture.

There are some basic symmetry patterns found in animal groups:

Radial symmetry is a "pie-slice" pattern where animals are roundish, and any radial slice (of a certain size) holds the same structures as any other.  Radial symmetry is often numeric - sea anemones are octaradial, so eighths are equivalent, starfish are pentaradial, in fifths.

Bilateral symmetry is two-sided.  These animals have set directionality - they move through the world with a front (anterior) end and a rear (posterior) end, an upper (dorsal) surface and lower (ventral) surface.  Nervous systems in bilateral animals show cephalization, a concentration of receptors and processors at the anterior end.

A common architectural plan is metamerism, with more-or-less repeating segments between specialized anterior and posterior ends.   Each segment / metamere repeats nervous, muscular, excretion, bristles & paddles, and primitively reproductive structures.  The digestive system is not metameric, and the nervous system has sense organs and special bundles anterior and posterior.  This is a simple approach to building a larger animal, and even in animals with reduced metamerism, it is an important part of embryo development.

Introduction to the Major Groups

These are the major animal phyla that will be discussed in the later chapters, with a brief description and some reproductive and developmental features -

Porifera.  The sponges.  This group are filter feeders, a common mode of feeding that involves straining food (in this case, microscopic organisms) out of the water.  Sponges are sessile, they stay in place.  They have two cell layers, an outer epithelium, and an inner layer of mostly choanocytes, filtering cells.  Between the layers is a jellyish mesoglea, with structural spicules of various materials and roaming amebocytes, which can replace and become any dead or lost cells.  Do not technically have tissues.  Various types of symmetry.

Cnidaria.  Includes jellyfish, corals, anemones, Hydra, etc.  Unique for stinging cells in ectoderm.  Have ectoderm and gastroderm around a digestive space, with mesoglea and amebocytes.  Also without tissues, although distinction is less clear.  Are radially symmetrical, often numeric.  Alternation of generations is common, with sessile asexual forms (polyps) and mobile sexual forms (medusas, small jellyfish).  May be colonial.

Platyhelminthes, flatworms.  Includes free-living planaria, parasitic flukes and tapeworms.  Have mesoderm and complex organ systems, no body cavities.  Parasitic groups use alternation of generations, with asexual form typically in one host species (intermediate hosts) and sexual form in another (definitive hosts), sometimes with vector hosts between.  Typically monoecious.

Nematodes, roundworms.   Found in almost every niche usable by a cylindrical worm.  Have acoelomate body cavity.  Have digestive system with mouth and anus.  Typically sexual and dioecious.  Adults often have set number of cells.  Almost protostomes.

Trochophore larval forms are found in many of these mid-list phyla, as well as minor phyla not listed.

Mollusks.  Includes gastropods, the snails and slugs, bivalves, the mussels, and cephalopods, the octopus, squid, and cuttlefish.   Protostome group.  Body form has foot in different forms, visceral mass with coelom, and mantle, which often generates a shell and surrounds a multi-function mantle cavity.  Some appearance of metamerism.

Annelids, the segmented worms.  Includes polychaetes, marine worms, oligochaetes, fresh water & earthworms, and hirudinea, leeches.  Have definite metamerism, reduced in leeches.   Protostomes.

Arthropods, many varied animals with exoskeletons. There are two subgroups:  the chelicerates, including spiders, horseshoe crabs, and scorpions;  the mandibulates, including crustaceans, insects, millipedes, and centipedes.  Most have reduced metamerism.  Protostomes.

Echinoderms, including starfish, sea urchins, and sea cucumbers.  This is a deuterostome group.  Most are pentaradial, although bilateral as early embryos (and sea cucumbers are pentaradial as late embryos but then become largely bilateral).

Chordates, including vertebrates, backboned animals.  Technically metameric deuterostomes.  Have notochord, structural in primitive forms but an inductive structure in early embryos in most.  Sealed eggs with amnions, structures that allow gas exchange with minimal water loss, found in land forms (reptiles, birds, primitive mammals).  Umbilicus attaches embryo to yolk (and mother in most mammals).

KEY CONCEPTS -
Click on term to go to it in the text.
Terms are in the order they appear.

 Offspring Numbers
Asexual Reproduction
Asexual Adaptation
Asexual Modes
Fission
Budding
Fragmentation
Regeneration
Testes
Sperm Production
Egg Cell Production
Accessory Organs
Primary Sex Features
Secondary Sex Features
Gender Determination
Monoecious Usefulness
Alternation of Generation, Animals
Fertilization
Fusion
Polyploidy, Animals
Mitochondria & Fertilization
Fertilization Types
Protostomes
Cleavage Patterns
Spiral Cleavage
Deuterostomes
Radial Cleavage
Determinant - Nondeterminant
Blastula
Ectoderm
Endoderm
Mesoderm
Coelom
Organization Levels
Tissues
Epithelium
Connective Tissue
Muscle Tissue, Subtypes
Nervous Tissue
Sense  Receptors
Sensory Neurons
Interneurons
Motor Neurons
Synapses
Neurotransmitters
Reproductive Tissue
Layout Patterns
HOX Genes
Symmetry Patterns
Radial Symmetry
Bilateral Symmetry
Anterior
Posterior
Dorsal
Ventral
Cephalization
Metamerism
Porifera
Filter Feeders
Sessile
Choanocytes
Mesoglea
Spicules
Amebocytes
Cnidaria
Gastroderm
Polyps
Medusas
Platyhelminthes
Nematodes
Trochophore Larva
Mollusks
Gastropods
Bivalves
Cephalopods
Foot
Visceral Mass
Mantle
Annelids
Arthropods
Chelicerates
Mandibulates
Echinoderms
Pentaradial
Chordates
Vertebrates
Notochord
Amnion
Umbilicus

GO ON TO THE NEXT CHAPTER - ANIMAL STRUCTURES