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A cryptic species complex in the ant parasitoid Apocephalus paraponerae (Diptera: Phoridae) by Shellee Morehead, Jon Seger, Don Feener and Brian Brown |
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Cryptic species are distinct in all the usual ways except that they look
the same even to experts. Prior to about 20 years ago cryptic species
were thought to be uncommon, for the obvious reason that they were not
easy to recognize. But the increasingly widespread application of
molecular genetic methods to studies of population structure and
systematics has revealed that many long-established "species" are really
two or more fully isolated and ecologically differentiated forms. We
recently discovered that a well known and well described ant-decapitating
parasitoid fly, "Apocephalus paraponerae" is in fact a complex of
at least four species. Even though these sister species are virtually
identical morphologically, they appear to have been separated from each
other by as much as seven million years. This finding supports the
emerging view that species diversities may be much higher than previously
believed, at least in certain taxonomic groups. |
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Apocephalus paraponerae males and females mate, feed and oviposit
on injured worker ants. Larvae develop inside the ant, migrate to the
head and pupate there, eventually causing the head to separate from the
thorax. The flies seen here have been attracted to an injured
Paraponera clavata, but populations of this "species" are also
attracted to ants in the genera Ectatomma and
Pachycondyla. |
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We began to suspect that flies attracted to Paraponera and
Ectatomma might belong to different species when we noticed that
individuals collected from Ectatomma workers did not respond to
4-methyl-3-heptanone and 4-methyl-3-heptanol. These compounds are
produced in the mandibular glands of P. clavata, and we had
previously shown that they serve as long-range olfactory cues for flies
attracted to P. clavata. Additional behavioral experiments at
study sites in Costa Rica and Panama confirmed that flies associated with
the two host ant species respond to different chemical cues, and a careful
morphometric study showed that flies associated with E.
tuberculatum are slightly but significantly smaller, on average, than
those associated with P. clavata, even though both ants occur and
attract "A. paraponerae" at both study sites. To test the implication that there are actually two distinct populations of flies, each specialized on a different host ant, we sequenced a portion of the mitochondrial COI gene from several individuals attracted to each ant species at each study site, and also from individuals attracted to Paraponera in Colombia and Ecuador, and to Pachycondyla in Colombia. We were not surprised to find substantial genetic differences between the flies attracted to the three different ants, but we were very surprised to find differences just as large between some populations of flies attracted to the same host ant, as summarized in the figure below. |
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Relationships and average genetic distances of "A. paraponerae"
collected from three species of host ants at various locations in Central
and South America. Several individuals from each collection were
sequenced, and they never showed more than minor differences. The scale
of distance (Ks) is synonymous substitutions per synonymous nucleotide
position. |
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The four populations of Paraponera flies form a monophyletic group,
but the Central American populations appear to be well separated from the
South American populations. The biggest surprise by far was finding that
Ectatomma flies in Costa Rica are only distantly related to
Ectatomma flies in Panama, which are more closely related to all of
the Paraponera flies and to the Colombian population associated
with Pachycondyla. The genetic distances among these apparently
distinct populations suggest that they have been separated for times on
the order of one to seven million years, based on calibrations of the COI
molecular clock for other Diptera. Many recognized species show distances
no greater than these, so we infer that these populations of "A.
paraponerae" represent at least four different species. In retrospect, it makes sense that Apocephalus should readily speciate, because their ecology includes all of the features believed to promote host-race formation: they meet, mate, and reproduce at a well defined host resource, and they use highly specific chemical cues to locate the host. Thus a host switch can give rise immediately to strong reproductive isolation and then to selection for improved performance on the new host. It will be interesting to see how many more cryptic species of Apocephalus there really are! Morehead SA, Seger J, Feener DH Jr, Brown BV (2001) Evidence for a cryptic species complex in the ant parasitoid Apocephalus paraponerae (Diptera: Phoridae). Evolutionary Ecology Research 3:273-284 |
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Related poster Do island endemics suffer increased genetic loads? by Kevin Johnson and Jon Seger To Jon Seger's research interests To Donald H. Feener Jr.'s research interests To a list of all posters in the session |