What’s in a Clone: The Rapid Evolution of Aphid Asexual Lineages in Relation to Geography, Host Plant Adaptation and Resistance to Pesticides

Abstract

The term “clone”, coined over a hundred years ago (see Chapter 9), is still in common parlance and widely used throughout the world, but its usage depends on definition, which is still obscure, especially if this involves the concept of genetic fidelity between clone mates rather than just the offspring from an asexual female founder (more correctly, an asexual lineage). To date, there have been no DNA sequencing studies proving such fidelity; on the contrary, the various DNA molecular marker studies performed on aphids display widespread genetic variation within and between different clonal lineages, as expected since mutation is a fundamental property of the DNA and hence the genome itself. In this overview, I use aphids as a model system to show that, rather than being an unchanging evolutionary “dead end”, asexual aphid lineages show rapid and widespread adaptive changes to changing ecological conditions in the field, including in relation to geography, host plant factors, and to insecticide applications. This being so, the so-called clone cannot be a fixed entity in time and space, but like all other living organisms in the real world, is evolving in response to its environment.

Glossary

Androcyclic:

asexual aphid lineage that produces mostly males and some asexual females.

Anholocyclic:

permanently viviparous asexual aphid lineage reproducing by apomictic parthenogenesis. This term can include an obligate asexual lineage or species, or a “facultative” holocyclic lineage of a species in which temperature and light conditions are favourable to maintain all year round parthenogenetic propagation (i.e., 16 h L: 8 h D and > 15°C ambient temperature), rather than induce the production of the pre-sexual and sexual forms (i.e., 8 h L: 16 h D and ambient temperature < 15°C).

Cline:

here used in the sense of a gradual change in allele or genotypic frequencies at a given locus/loci, across the distributional range of a species population, and correlated with an environmental/ geographic transition.

Gynoparae:

winged pre-sexual aphid female morph produced under shortening day, cooler ambient temperature conditions from a holocyclic asexual lineage, along with males. It migrates to a new host where it produces sexual females (oviparae) which in turn mate with the males to produce cold hardy overwintering egg. The new host could be a primary woody one in the case of host alternating aphid species:

Holocyclic:

an aphid lineage and/or species with an annual sexual phase, usually involving an autumnal winged migration. In the case of species that have an alternation of plant hosts, this is from a secondary herbaceous host to the primary woody host and vice versa, but there are exceptions (e.g., Sitobion avenae which remains on Poaceae all year round). Such holocyclic forms can also be “facultative” anholocyclic.

Ovipara:

egg laying sexual aphid female. This morph attracts the males (winged in most species, but there are exceptions) using sex pheromones.

Telescoping of generations:

the phenomenon whereby asexual aphid females have both their offspring (children) and their offspring (grandchildren) within them.

Virginoparae:

asexual viviparous female aphids which produce further such asexual females.

Wheat volunteers:

cultivated wheat sprouting from the seeds of plants sown in previous sowings.