* Virgin female reproduction (parthenogenesis) has been studied
extensively, notably by Dr. Warwick Kerr.

* It is worth noting that the race primarily known for this trait is
also considered a disastrous pest, reinforcing the idea that asexual
reproduction is harmful in the long run.

> One known case of somatic parthenogenesis in the honey bee (Kerr unpublished) suggests automictic parthenogenesis. This type of somatic parthenogenesis requires meiotic chromosome reduction, as opposed to apomictic parthenogenesis in which meiosis is completely absent. Kerr obtained results denoting meiotic chromosome reduction; these were the appearance of two homozygous recessives among eight impaternate worker bees from heterozygous unmated queens.

> Occasionally worker honeybees develop ovaries and lay unfertilized eggs. Usually these are haploid, as you would expect, and develop into males. However, workers of the subspecies Apis mellifera capensis (the Cape honeybee) can lay unfertilized diploid eggs that develop into females (who continue the practice). The eggs are produced by meiosis, but then the polar body nucleus fuses with the egg nucleus restoring diploidy. (The phenomenon is called automictic thelytoky.) (1)

* * *

Why choose asexual reproduction? Perhaps the better question is: Why
not? After all, asexual reproduction would seem a more efficient way
to reproduce and avoids all sorts of problems. Perhaps sexual
reproduction has kept in style because it provides a mechanism to weed
out harmful mutations that arise in the population (through the
recombination process of meiosis).

Evidence (from Paland and Lynch in the 17 February 2006 issue of Science):

Some strains of the water flea Daphnia pulex (a tiny crustacean)
reproduce sexually, others asexually. The asexual strains accumulate
*deleterious mutations* in their mitochondrial genes four times as
fast as the sexual strains.

But there are many examples of populations that thrive without sex, at
least while they live in a stable environment. Perhaps it is the
ability to adapt to a changing environment that has caused sex to
remain the method of choice for most living things.

Evidence (from Goddard et al. in the 31 March 2005 issue of Nature):

Budding yeast missing two genes essential for meiosis adapt less
rapidly to growth under harsh conditions than an otherwise identical
strain that can undergo genetic recombination. Under good conditions,
both strains grow equally well.

An asexual population tends to be genetically static. Mutant alleles
appear but remain forever associated with the particular alleles
present in the rest of that genome. Even a beneficial mutation will be
doomed to extinction if trapped along with genes that *reduce the
fitness* of that population.

But with the genetic recombination provided by sex, new alleles can be
shuffled into different combinations with all the other alleles
available to the genome of that species. A beneficial mutation that
first appears alongside harmful alleles can, with recombination, soon
find itself in more fit genomes that will enable it to spread through
a sexual population.

Evidence (from Rice and Chippindale in the 19 October 2001 issue of Science):

Using experimental Drosophila populations, they found that a
*beneficial mutation* introduced into chromosomes that can recombine
did -- over time -- increase in frequency more rapidly than the same
mutation introduced into chromosomes that could not recombine.

So sex provides a mechanism for testing new combinations of alleles
for their possible usefulness to the phenotype:

    * deleterious alleles weeded out by natural selection;
    * useful ones retained by natural selection. (2)


SOURCES:

(1) "Automictic Parthenogenesis In The Honey Bee"
Kenneth W. Tucker
Department of Entomology and Parasitology
University of California, Davis


(2) "Asexual Reproduction"
Dr. John W. Kimball
Phillips Academy
Andover, Massachusetts

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