Biology Reference
In-Depth Information
juveniles) or ingestion of larvae through predation on intermediate hosts
(small mammals and birds). 10 Accidental ingestion of B. procyonis eggs by
humans in areas where racoons and humans live in close proximity can
cause a form of visceral, neural or ocular larva migrans. 10 e 14 Nonetheless,
it is likely that the host range of many of these worms is catholic, open to
evolutionary opportunity when given sufficient epidemiological potential.
An evolutionary quirk of Parascaris, shared with Ascaris, is that nuclear
DNA is greatly reduced during early development in all somatic cells but
not in germ-line cells. This developmentally-regulated DNA rearrange-
ment, known as chromatin diminution, is highly specific with respect to
timing and chromosomal location. 15,16 These non-Mendelian processes
have frustrated the application of genomic fingerprinting techniques such
as RAPDs (randomly amplified polymorphic DNA) and therefore one
must always remain aware of unusual molecular drive processes that
confound molecular systematic approaches. 17 Interestingly, sequence
analysis of chromosomal breakage regions (specific sites where elimina-
tion of DNA takes place) has shown that these regions are conserved
between Ascaris and Parascaris, suggesting that these sites were present in
a common ancestor of these parasites. 18
SPECIES CONCEPTS
The distinction between A. lumbricoides and A. suum and whether they
should be regarded as one or two species still engenders debate today. 19,20
We will discuss this issue below but for simplicity and convenience we
will use A. lumbricoides and A. suum to refer to human and pig Ascaris,
respectively. We now briefly review the more common species concepts
which are broadly applicable. The conclusion one reaches on whether
there are one or two Ascaris species is, of course, highly influenced by the
species definition used and this issue is often ignored in the debate.
Species is the lowest taxonomic unit used in biology to classify organisms
into taxonomic ranks but a range of different species concepts have been
proposed largely tailored to a generalized concept of “organism” which
does, of course, differ in application across the Metazoa. 21
The major classes of contemporary species definitions include the
phenetic species concept which groups organisms that are phenotypically
similar and can be distinguished from other organisms into a coherent
phenetic cluster. 22 Thus if no morphological differences can be found
between Ascaris obtained from humans and pigs they must be considered
a single operational taxonomic unit (OTU) or “one” species. The phylo-
genetic species concept typically takes a cladistic approach where a species
can be defined as an irreducible clade of organisms that is diagnostically
distinguishable from others by analysis of synapomorphic characters
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