Reproductive Strategies of Nematodes

Rev. 09/03/2017

I. Parthenogenesis

(a) Types

Meiotic (involves a reduction division) and mitotic (no reduction division).

(b) Definition

Mating and genetic recombination do not occur.

Except for mutations, mutations, offspring should be genetic clones of parent.

(c) Mechanisms

In Mitotic Parthenogenesis, the diploid chromosomes replicate themselves and the nucleus separates into two daughter nuclei, each with a full complement of daughter chromosomes.  Cell division follows.  There is no possibility of change in the genetic material, the ova should be clones unless a mutation occurs.

In Meiotic Parthenogenesis, a reduction division occurs.  The chromatids of each diploid pair separate, one member of each pair remains in the nucleus while the other is confined to a polar body near the cell membrane.  The resulting haploid ovum may be fertilized by a male sperm as it passes through the spermatheca if copulation has occurred.  Otherwise the nuclear chromosomes will pair again with those in the polar body to form the diploid complement.  In the case of nuclear chromosomes reuniting with those from the polar body, the ova should be clones.  Genetic recombination will occur in those cases where fertilization occurs in the spermatheca.

(d) Sex determination

The chromosome replicating process of Mitotic Parthenogenesis and Meiotic Parthenogenesis where fertilization does not take place, determines that all offspring will be genetically female with two X chromosomes.  Sex determination is as yet uncertain as males can occur in these populations and their numbers may increase under adverse conditions.  It seems highly likely that sex may be determined by the effect of environmental conditions on the degree to which genes from the X chromosome regulate genes on other chromosomes.  That would allow for environmentally-mediated sex determination, which is frequently observed.  For example, since in sexually reproducing species females are XX and males are XO, one might infer that more X product is required to up- or down-regulate the genetic pathways that result in females than for those that result in males.  Consequently, if the signal strength from the X chromosomes was suppressed at high temperature, more males might result.

(e) Advantages

(i) Individual level - No need to locate a mate.

(ii) Population level - High rate of increase, all adults produce offspring, very strong adaptation for opportunism in a disrupted environment.

(f) Disadvantages

(i) Individual level - Competition for resources at high population levels.

(ii) Population level - Change in genotype based on mutation rather than mutation and recombination - perhaps a narrower basis for selection in a changing environment.

(f) Examples of genera or species

Meloidogyne, etc.

II. Amphimixis (Sexual Reproduction)

(a) Types

Obligate and facultative.  Facultative amphimixis is the equivalent of the mechanism described under meiotic parthenogenesis.

(b) Definition

Mating and genetic recombination.

(c) Mechanisms

Reduction divisions from 2n chromosomes to n chromosomes (meiosis) occur in the development of both spermatocytes in the male testis and oocytes in the female ovary.  Copulation occurs, sperm move through the uterus where they may fertilize eggs or into the spermatheca where they fertilize the ova as they pass through.  The nuclei of ovum and sperm unite to regenerate the haploid condition.  Recombination of genetic material occurs in the process.

(d) Sex determination

Sex determination is as yet uncertain.  We know that nematode females are XX in the sex chromosome and males are XO.  Clearly both these conditions could occur as a result of amphimixis and sex determination could be genetically controlled.  However, it seems highly likely that sex may be determined by the effect of environmental conditions on the degree to which genes from the X chromosome regulate genes on other chromosomes.  That would allow for environmentally-mediated sex determination, which is frequently observed.

(e) Advantages

(i) Individual level - Slower population growth as some portion of the population becomes males;  that is, all adult individuals do not produce eggs.  This may be an advantage to the "quality of life"  of individuals at high population levels.

(ii) Population level - Genetic recombination, heterogeneous range of genotypes as a basis for selection in a changing environment.

(f) Disadvantages

(i) Individual level - Need to find mate - may be difficult depending on population level and nature of parasitic habit.

(ii) Population level - Reduced rate of population increase due to mating constraint and that a significant portion of the population (males) does not produce offspring. Perhaps less of an adaptation to the opportunism and "r-strategy" characteristics that might confer success in the changing and disruptive conditions of intensive agriculture.

(g) Examples of genera or species

Heterodera, etc.

III. Hermaphroditism

References

Gems, D. and D. L. Riddle. 1996. Longevity of Caenorhabditis elegans reduced by mating but not gamete production. Nature 379:723-725.

Kimble, J. and S. Ward. 1988. Germ-line development and fertilization. In: W. B. Wood et al. The Nematode Caenorhabditis elegans. Cold Spring Harbor Laboratory Press.

Williamson, V.M. 2001. Personal communication.

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