The ecology of the subspecies of the pea aphid.
The pea aphid (Acyrthosiphon pisum) was one of the first aphid species for which
different biotypes were described. Subsequently, the differences between biotypes were
found to be consistent in time and space and several of them were given subspecies
status. The differences between the subspecies is mainly ecological, their use of certain
plants (the so-called marker hosts). There are hardly any differences in the morphology
of the subspecies with the exception of that from Restharrow (Ononis spec. ).
The performance and survival of aphids on several host plants were used to determine
the degree of separation between the pea aphid subspecies and their marker hosts. To
confirm the genetic basis of the host plant relations of the subspecies they were crossed.
Few of the crosses showed hybrid dysfunction. The performance and survival of the
hybrid clones confirmed that host plant relationships were genetically determined.
There was also indication of a trade off. However, there was no indication that
"Hopkin's host selection principle" played a big role in the utilisation of non-preferred
host plants, with possible exception of clover.
The different taxa differ significantly in body sizes. Clones from crop plants were
generally bigger than those from wild plants. The genetic component of the size
difference accounted for nearly 50 percent of the variances in size in wild clones.
By comparing the performance of reciprocal crosses between subspecies on the marker
hosts of the parents, no evidence was found that the specialised symbionts are
specialised for particular marker hosts. This indicates that the aphid's genotype is the
main determinant of host plant usage in the pea aphids.
Furthermore, these aphids prefer their respective marker hosts. The connection between
preferencea ndp erformancew as partly broken by hybridising the subspecies.
The only subspecies that produces winged males and therefore has the ability to
colonise other host plants, and thus the opportunity to mate with females of other
subspecies, preferred sexual females of its own subspecies. The separation of the
subspecies is further enhanced by the behaviour of egg laying females, which preferred
to oviposit on their marker hosts. Hatching time of the eggs was also associated with
the ecology of their marker host plants and probably the life history of the aphid, i.e. the
subspecies that host alternates hatched first.
The ecological separation between the subspecies was not confirmed by a molecular
analysis, which even failed to separate the morphologically distinct subspecies from
Ononis from the others taxa. The pea aphid complex is a good example of sympatric
taxa, which is isolated from one another by their preference for particular marker hosts.
That is, host plant is the main pre-zygotic separation mechanism, which is likely to lead
the development of post-zygotic separation mechanism and eventually to fully