jimtrue.com : school : BSC2011 : CH 46: Animal Systems - Reproductive (pp 975-998)

Posted by Jim True on December 1, 2004 7:32 PM. Last Updated October 22, 2006 9:23 PM

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CH 46: Animal Systems - Reproductive (pp 975-998)

Function

• Overall function is to carry out the biological imperative - replication of self and the perpetuation of the species!

Reproductive Modes

• Two reproductive modes are:
1. Asexual(AR) - Inheritance of all genes and production of offspring from one parent. ALWAYS REPRODUCTION BY MITOSIS & CYTOKINESIS!
• Under normal conditions, offspring is/are genetically identical to parent. "Clone"
• Asexual reproduction can occur in many of the simpler animal groups.
2. Sexual Reproduction (SR) - Joining of two cell nuclei each brining 1/2 of the genetic information contained by the parent cells.
• Usually, but not always, two different parents.
• These two nuclei are contained within specialized cells (the gametes).
• In animals these cells are produced by meiosis within multicellular structures called gonads.
• In males, the gonad is the testis (plural - testes). Spermatozoa ("sperm") are the gametes produced in the testis.
• In females, the gonad is the ovary (plural - ovaries). An ovum (plural - ova) is the gamete produced in the ovary.

Asexual Patterns

• Among different animal groups, a variety of asexual patterns are observed:
• Fission ("splitting") - Simplest form. Organism literally divides into two new smaller organisms of approximately equal size.
• Budding - In animals, a localized mass of cells undergo mitosis and produce a new, smaller offspring.
• Gemmulation - specialized asexual producing in phylum Porifera (esp. Fresh water) consisting of functional cells within a protective capsule (gemmule). Develops during poor environmental conditions.
• Fragmentation - Pieces of "parent" organism break off and develop into new individuals. Parent organism undergoes regeneration, mitotic replacement of missing piece(s).
• Parthenogenesis ('virgin birth") - Production of a diploid egg from diploid mother cell, or haploid individual from haploid egg -- NO FERTILIZATION INVOLVED!
• The advantages of asexual reproduction include:
• Rapid population growth.
• Individuals can be isolated since they do not need to locate mates.
• A successful adaptationn for one is successful for all
• Disadvantages include:
• Lack of genetic varability
• Possible inability to respond to environmental changes (what harms one, harms all).
• Asexual reproduction is favored in stable environments or in variable environments during times of relative stability.

Sexual Patterns

• Patterns include both the anatomy and physiology of the parents in terms of gamete production and means of combining gametes, as well as the condition of the offspring at birth.
• Patterns of Gamete Production
1. Monoecious - AKA Hermaphroditism. May be synchronous or simultaneous.
• Synchronous (AKA Sequential) - Individual changes gender during life.
• Protogyny ("first female") Individual begins life as female, changes to male.
• Protandry ("first male") Individual begins life as male, changes to female.
• Simultaneous - Individual has fully functional male and female gonads at the same time. Self-fertilization is rare, however.
2. Dioecious - Males and females are separate individuals.
• Sexual dimorphism - In many dioecious animals, males and females differ in appearance. Maybe size, shape, structures, color, etc.
• Patterns of Gamete Mixing and Offspring Production
• Oviparity - Ova are shed from female body for fertilization. Males release sperm externally.
• Fertilization and development are external to parents bodies.
• In some cases, parental care of developing offspring may be exhibited, but usually no care.
• Ovoviviparity - ("ovo" - egg; "vivi" - living) - Ova are fertilized internally, offspring develop within female (or male) but without parent providing nutrition.
• Requires physical contact between individuals and often anatomical adaptations to allow sperm to be introduced inside females body.
• May or may not involve parental care once young are born.
• Viviparity - Eggs fertilized internally, develop within female body and are nourished by the female.
• Also requires physical contact and introduction of sperm into the female body.
• In ovoviviparity and vivparity, offspring are born alive.
• Viviparous young typically involve a period of parental care, sometimes for very long periods (years).
• Sexual reproduction is more typical of environments that are unstable or highly variable, also when conditions are unfavorable.
• Advantages of sexual reproduction include:
• Genetic variability; greater adaptability to environmental change
• Development of social structure and interactions
• Disadvantages include:
• Search for mate costs energy and time, potential exposure to predator
• Potentially advantageous characteristics for specific environment may not be represented in the next generation.

Systems

• P. Porifera Asexual reproduction occurs via budding or fragmentation.
• No gonads for sexual reproduction. Instead, cells transform into sperm and ova (chaoanocytes produce sperm, amebocytes or choanocytes produce ova).
• Many species are viviparous. (Intracellular digestion also is fed to the developing embryos)
• Larvae (parenchymula) are flagellated and free swimming for a period, then settle down and invert so that flagellated outer cells become inner choanocytes.
• P.Cnidaria - Two of the three classes exhibit alternation of generations; asexual polyp and sexual medusa.
• The polyp typically reproduces by budding.
• Medusae are typically dioecious and fertilization may be external or internal.
• Young may be brooded, held on or in special region of the body during early development.
• Medusae produce ciliated planula larvae which settle and transform into polyps.
• Exception to this is the C. Anthozoa, which has no medusa stage.
• Asexual reproduction is by fission, budding or fragmentation.
• Sexual reproduction - some species are dioecious, others are protandrous hermaphrodites.
• Fertilization is external or internal, and produces a planula larva.
• P. Platyhelminthes - Reproduction varies among classes.
• C. Turbellaria - Asexual - fission.
• Sexual - Monoecious, simultaneous hermaphrodites that cross-fertilize.
• Exhibit copulation, physical contact between individuals to ensure sperm contact with ova.
• Development is external and occurs within a cocoon, protective capsule around embryo.
• C. Trematoda and Cestoidea - Both exhibit only sexual reproduction.
• Members of both classes are typically monoecious and simultaneous, although typically exhibit cross-fertilization.
• Most elaborate life cycles of any animals.
• There may be two or more different larval stages, each of which is specific to a different host species, with adults in a different final host.
• Indirect development - when larval forms(s) differ(s) in appearance from adult, usually accompanied by metamorphosis, transformation from an organism bearing no resemblance to adult in adult form.
• P. Nematoda - Sexual reproduction only.
• Dioecious and exhibit size dimorphism (males are smaller).
• Copulation occurs - males bear copulatory spicules which hold genital pore of female open against hydrostatic pressure.
• Unique to animal kingdom, nematodes produce ameboid sperm. Allows sperm to crawl to ova against hydrostatic pressure.
• Fertilization is internal.
• Development is direct - young look like miniature adults.
• Parasitic adults may have free living or parasitic larval stages.
• P. Mollusca - Sexual reproduction only; varabile patterns by class.
• C. Bivalvia - Usually dioecious, with external fertilization in marine forms; internal in freshwater species (in freshwater, there are usually currents and the sperm could possibly wash away)
• Produce ciliated trocophore larva in marine environment, but a specialized parasitic larva (glochidium) in freshwater.
• C. Gastropoda - Includes both dioecious and monoecious forms, with both external and internal fertilization.
• Copulation occurs in most and development is either external or ovovivparous.
• Some gastropods exhibit courtship rituals prior to mating.
• C. Cephalopoda - Species are dioecious and dimorphic.
• Males have a specialized tip of one arm used to insert sperm packets into the mantle of the female
• Copulation and internal fertilization occurs in all.
• Elaborate courtship rituals are known for many species.
• Development of young is external, although some species guard their eggs during direct development.
• P. Annelida - Asexual by budding (a few families in C. Polychaeta only).
• Sexual - dioecious (C.Polychaeta) or monoecious, simultaneous, cross-fertilizing (C.Oligochaeta and Hirudinea).
• In polychaetes, gonads are not discrete, but develop as masses of gametes that are shed into the coelom.
• Fertilization is external and produced ciliated trocophore larva.
• Copulation occurs in oligochaetes and leeches.
• When sexually mature, oligochaetes (always) and leeches (seasonally) develop a glandular region, the clitellum.
• This secretes a cocoon for direct development of young.
• P.Arthropoda - Highly variable among the subphyla.
• S.P. Chelicerata - also variable depending on whether the species is terrestrial or aquatic, but typically all are dioecious and most exhibit size dimorphism.
• Development may be indirect or direct, depding on the group.
• S.p. Crustacea - Some species are monoecious, simultaneous cross fertilizers and parthenogenesis occurs in members of several classes.
• Most species are dioecious and may exhibit sexual dimorphism
• Copulation is typical for most and development is external. Many species brood their young on or in special regions of the parent body.
• Development may be indirect or direct, depending on the group.
• S.p. Uniramia - All are dioecious.
• Copulation occurs and most forms release fertilized eggs to environment, but may exhibit some parental care (millipedes and social insects, such as ants and bees).
• Most insects (in fact, most arthropods) exhibit elaborate courtship rituals.
• Pheromones, chemicals to attract opposite gender often used in mate location.
• P. Echinodermata - Asexual reproduction is possible in some via regeneration (Asteroidea, Ophiuroidea). Depends on how much of the central body remains in the fragmented portion.
• All classes have external fertilization, and except for the Holothuroidea, which includes some hermaphrodites, all are dioecious.
• All produced bilateral ciliated larval stages that undergo metamorphosis included complete shift of symmetry. Each class has a diagnostic larval type.
• P. Chordata, S.p. Vertebrata - All exhibit sexual reproduction, although some cases of parthenogenesis are known among certain species of Osteichthyes.
• Reproductive and developmental modes include ALL variations presented.
• Dimorphism is not uncommon among dioecious species and includes many variations.

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