Evolution__3rd_Edition

196 PART 2 / Evolutionary Genetics
Genetic crosses initially suggested
one locus was at work ...
. . . but it turned out that at least
five loci were
mimic; this has been done for the nigra morph of P. glaucus, but not for P. memnon.
However, we can accept as a working hypothesis that the apparently mimetic morphs
of P. memnon indeed are mimetic. (P. memnon has yet further, non-mimetic forms too,
but they are not essential to the discussion here.)
Clarke and Sheppard were interested in the genetic control of this complex mimetic
polymorphism. Crosses between the various morphs initially suggested that a single
genetic locus, with many alleles, is at work. When two morphs are crossed, the offspring
usually only contain individuals of one or other parental phenotype. This is the
result expected if one locus is at work, with simple dominance relations among alleles.
For instance, if one morph has genotype A 1 A 1 , another A 1 A 2 , and A 2 is dominant to A 1
then an A 1 A 1 × A 1 A 2 cross produces the same two classes of offspring (A 1 A 1 and A 1 A 2 )
as were present in the parents.
But the genetic story soon grew more complicated. In addition to the mimetic and
non-mimetic morphs of P. memnon, all of which exist in reasonable frequencies in
nature, some much rarer morphs have been found. An example, in Java, is the rare
morph called anura (Plate 3m). A specimen found in Borneo was sent to Clarke
and Sheppard in Liverpool. When it was crossed with a known P. memnon morph, it
behaved like another allelic form of the mimicry “locus”; but a closer look at anura
suggests a different interpretation. Anura’s morphology mixes patterns from two of
the common morphs: it has the wing color pattern of the morph achates (Plate 3g–i),
but it lacks achates’ tail.
Clarke and Sheppard’s interpretation is that anura is not an allelic variant, but a
recombinant, and that the mimetic patterns of P. memnon are not controlled by one
locus but by a whole set of loci. If anura is a recombinant, then there must be at least
one locus (call it T) controlling the presence (allele T + ) or absence (T – ) of a tail and at
least one other locus (C) controlling the color patterns (C 1 for achates, and C 2 , C 3 , etc.,
alleles for other color morphs). Achates would have a genotype made up of one or two
sets (depending on whether the alleles are dominant) of the two-locus genotype T + C 1 ,
and anura would have T – C 1 , after recombination between a tailless morph and achates.
The loci in question are so tightly linked that these recombinants practically never arise
in the laboratory a which is why the different multilocus genotypes appear, when
crossed, to segregate like single-locus genotypes. We can predict that if more than one
locus really is involved, a sufficiently large number of crosses should be able to break
one of the “alleles” (such as the anura “allele”) into several real combinations of alleles
at several loci.
From anura alone, at least two loci could be inferred to control the mimetic polymorphism
of P. memnon; but other rare morphs have also been found. Some rare
morphs, for example, combine the forewing color of one morph and the hindwing
pattern of another, suggesting that separate loci control the color of the fore- and hindwings.
When all the inferred recombinants are considered together, at least five loci
seem to be at work: T, W, F, E, and B. They control, respectively, presence or absence of
tail, hindwing pattern, forewing pattern, epaulette color, and body color. The anura
morph is a recombinant between the T locus and the other four. The common morphs,
which mimic natural models, should each consist of a particular set of alleles at the five
loci. The morph mimicking model species no. 1, for example, might have genotype
T + W 1 F 1 E 1 B 1 /T + W 1 F 1 E 1 B 1 , and another morph (mimicking a second model) might have
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