Autumn.wmf (12088 bytes)Introduction to Organismal Biology (BIOL221) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321;;

Reproduction:  A Primer

 "an individual is the transient caretaker of the genetic instructions that must be conveyed down the generations."
     H. Morowoitz

I. Goal
    According to Harold Morowitz, one of my favorite science writers (he’s a biochemist), reproduction can be viewed as the mechanism by which an individual passes genetic information through time.  So, how do plants and animals convey the genetic instructions from one generation to the next?

II. Plants & animals can reproduce sexually
Exactly what is "sex"? - biologically "sex" is defined as the process that involves meiosis and fertilization. Thus, during sexual reproduction meiosis ultimately results in haploid gametes that fuse or recombine during fertilization ( also called syngamy). 

  1. The basic sexual life cycle:
        adult (2n, diploid)
    meiosis gametes (n, haploid) fertilization zygote (2n) embryo adult
  2. Advantage of sexual reproduction
         Perhaps the major advantage of sexual reproduction is that the offspring are genetically different than either parent. Thus, this process gives rise to genetic variation, which is the key ingredient for evolution. 
  3. Plants & animals have "kinky" sex - in other words, there are many variations of the basic pattern described above.

III. Plants & animals can also produce offspring without sex (asexual reproduction)
    "A" means without; therefore asexual literally translates into reproduction without sex.  Thus, during asexual reproduction there is no production of gametes, no fertilization and hence, no genetic recombination. Obviously the offspring will be genetically identical to the parents (=a clone). Some points to ponder:

  1. Plants & animals can reproduce asexually by: 

    1.  Budding - producing new "offspring" directly from buds.  For example in plants, new individuals can arise from meristematic regions on modified: (a) stems such such as rhizomes (e.g., ginger),  stolons (e.g., strawberry); tubers (e.g., potato), bulbs (onion), corms (gladiolus); or (b) roots; or (c) leaves (e.g., maternity plant or mother-of-thousands).  In animals, hydra and yeast are classic examples of individuals that bud and a miniature version of the parent splits off. 

    2.  Parthenogenesis - offspring develops from an unfertilized egg.  In dandelions, seeds form without sex.  Similarly, many animals can develop from unfertilized eggs.

    3.  Fragmentation or fission - cells in plant tissues can differentiate into new individual (i.e., leaf cuttings); in animals - individuals split in half (e.g., flatworms, starfish)
  2. Plants excel at reproduction without sex; asexual reproduction is less common in animals.  However, some animal groups are also asexual champs.
  3. Asexually-produced offspring are "clones" or genetic carbon copies of the parents.
  4. Asexual reproduction is practicing "safe sex"
        In other words, asexual reproduction is a much safer bet for producing an offspring because an individual doesn't have to rely on the "chanciness" of the sexual process, or running into trouble (i.e., getting eaten during the process, catching a disease).
  5. A potential disadvantage
        In theory, asexual reproduction should limit genetic variation in a population, and hence, would be a disadvantage in the long-term evolutionary sense.
  6. Taking advantage of asexual reproduction
        Gardeners have long taken advantage of the asexual reproduction process in plants. If you’ve ever taken a cutting from a plant or grafted a plant, you have, too. Tissue culture is a high-tech version of the same process. Individuals cells of a plant can be grown aseptically in culture medium. Initially the cells form an undifferentiated cluster of cells, called a callus, which is essentially a plant "cancer."  These clumps can be divided up transferring them to other cultures producing literally thousands of genetically identical cell masses from an original cell. Then, when given the appropriate conditions and hormones, these calluses will differentiate into an entire new plant! Cool, eh? Using various techniques (a DNA gun or a bacterial vector) DNA carrying genes of interest can be inserted into the plant cells and new plants grown. My favorite example is the tobacco plant in which the gene for firefly luciferase was inserted. The plant glowed in the dark when watered with ATP.

IV. Why Reproduce Sexually?
    At first you might consider this a silly question, but consider:

  1. Sex is not necessary for reproduction
        Asexual methods (producing new individual without involving sperm, egg and fertilization), work well, are simpler, and less risky.
  2. Sex, in most species, is not for pleasure
        There is little foreplay in most species and orgasms are rare in other animal species. Also, consider fish and other species that simply shed gametes into the environment (external vs. internal fertilization).
  3. Sex is risky business
        With sex comes the increased, risk of disease, ectopic pregnancy, or getting eaten (sexual cannibalism - praying mantis & black widow stories).

    So, why sex? Because "Sex is not Selfish".  In other words, the result of sex is offspring that have different genes than their parents.  The fact that offspring are genetically different than their parents is a major advantage because it provides for genetic diversity that is the raw material for evolution.

V. Evolutionary Strategies 

A. Males vs. females
    For most species, males and females have different strategies when it comes to sexual reproduction. First, we will make the assumption that both sexes are interested in producing the maximum number of genetically fit individuals. So how do males and females optimize this possibility?

    Consider gametes - females produce few, large gametes whereas males produce numerous small ones. Since eggs are expensive and only a few offspring can be raised, the evolutionary pressure for a female is for "choosiness" in mate selection. In contrast, sperm are "cheap" and the evolutionary success of a male is limited by his ability to deliver sperm to the egg. Males have evolved to be "salesmen", attractive to females.  They advertise good genes by holding territories, displays, courtship rituals, gifts, appearance.

    Do humans follow these evolutionary ideas? There is considerable debate. Whadayathink?

B.  Female Gamete Choice
    Courtship and mating are not the only places females can exert control over breeding.  The females of several species are able to control which sperm fertilize an egg AFTER copulation.  These females have the ability to discriminate between sperm of different species as well as individual males of their own species (Birkhead, 2000).  For example, female fruit flies that mate with another species will not produce fertile eggs even though her sperm stores are full.  If artificially inseminated with the same sperm she will lay fertilized eggs.  Similarly, if mated with a closely related male and a distantly related male of her species, she will preferentially produce offspring from the distantly related species. 

C. Reproductive Output
    An individual man can theoretically have more children than an individual female. In fact, the maximum number of children for a female is about 20 whereas the most prolific father in history is presumed to be Ismail the Bloodthirsty, a Moroccan emperor from 1672 – 1727, who sired 888 children (see Natural History, Sept 1999, p 23).  This record is rather suspect even considering his harem of 500 women.  It's virtually impossible for him to have fathered all of the children attributed to him when you consider:

  1. the short period of fertility for a woman during her monthly cycle,
  2. the degree of infertile women in a group (about 10%);
  3. miscarriages and other problems (10-20%),
  4. generally low incidence of conception (15-40%), and
  5. a man's sperm count decreases with sexual activity.

    To father all of these children in face of these statistics he would have had to mate with 4.8 women daily for four decades. This performance makes Wilt Chamberlain look like a slacker. As an aside, even though males have the potential to have more offspring, in practice there isn’t great differences in human reproductive rates between males and females.

D. Monogamy
    If males and females have such differing evolutionary strategies, why isn't monogamy less common? Presumably because a monogamous coupling is paternity insurance for the male, since he can never be sure that the offspring is his.

    Are humans a monogamous species? Consider that for other monogamous species the females are usually more aggressive and larger than the males, are territorial, and show other features that we don't possess. As an aside, a study of blood types of parents and offspring done in the 1950's showed that about 10% of offspring born were fathered by someone other than the man presumed to be the father.

E.  Gender is relative
    Some plants in which there are separate male and female flowers on different individuals (e.g., Jack-in-the-pulpit) are able to change sex.  In other words, one year the plant will produce male flowers and the next female flowers.  Typically the first few years the plants are male and then about the seventh year they switch and produce female flowers.  Subsequent years are determined by the growing season of the previous season.  This allows the plants to maximize their reproductive output.  Some fish are also able to change sex!


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Last updated: April 16, 2009        � Copyright by SG Saupe