Antenna 31(4)
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ARTICLE Figure 4. Aphid (top right) as illustrated in a bookplate dating from 1805. These insects have been known to be pests of agriculture (more especially cereals) for at least 200 years. Their asexual spring and summer mode of reproduction, leading to their prodigious reproductive capabilities as noted in Part 1 of this article, Antenna (2007) Vol. 31(3), 152-158), was first shown by the Swiss naturalist and philosophical writer Charles Bonnet (1720-93) in 1740. However, the ability of particular aphid species to transmit pathogenic viruses to plants of economic importance (agricultural, horticultural and forestry) was only determined in the early years of the 20th century. The Myth of the Clone Big issues in Evidence for variation within the clonal genotype Some modern molecular evidence population biology: gives credence to clonal fidelity, other evidence does not. The use of microsatellite markers (Goldstein & lessons from a greenfly Schlötterer, 1999) has shown that aphids (Fig. 4) can have multilocus genotypes which remain consistent in Presidential Address terms of genotypic expression over a range of loci (12 tested; Haack et al., Continued from the July 2007 issue, Volume 31(3) 2000). This suggests that such multilocus genotypes have descended from a common stem mother (Haack Hugh D. Loxdale † et al., 2000; Miller, 2000). However, this may be an illusion of constancy. Royal Entomological Society, The Mini- and microsatellites, essentially Mansion House, Chiswell Green Lane, selectively neutral (but see Li et al., Chiswell Green, St. Albans, 2002), are known to be notoriously Hertfordshire AL2 3NS, U.K. fast mutating and evolving, with mutation rates typically of the order 10-6 to 10-3 per locus per generation † Address for correspondence: (Hancock, 1999). There are Institute of Ecology, Friedrich Schiller thousands of such loci, perhaps tens University, Dornburger Str. 159, 07743 of thousands, scattered throughout Jena, Germany (e-mail: the genome (Loxdale & Lushai, [email protected]) 2003a). In addition, even sampling 12 194 Antenna 31 (4) interclonal variation in the decline of gene expression, but not number and position of gene number, when chemical rDNA arrays in the selection ceases. I will allude to this chromosomes of aphids phenomenon at the end of the article belonging to the genus (see section ‘Was Lamarck partially Trama, of which males are right all along?’). Loxdale & Lushai unknown, although there (2003a) overview general mutational is other molecular changes in aphids whilst, more evidence for sexual specifically, Field & Blackman (2003) recombination events discuss the changes seen in the highly having occurred resistant strains of M. persicae in (Normark, 1999). relation to E4. With regard to genotypic banding Evidence for variation between clonal patterns as seen on gels lineages using predominantly Using microsatellite and mtDNA dominant markers markers, evidence for introgression (markers which usually do events between different lifecycle not provide heterozygous morphs within species as well as genotypes but are between closely-related species has nevertheless useful in been demonstrated. Thus for clonal studies; Loxdale & example, the two species S. avenae Lushai, 1998), new (predominantly anholocyclic) and S. mutated bands have been fragariae (predominantly holocyclic), Figure 5. Stack-bar graph (lower) and cladogram (upper) showing seen using synthetic both of chromosome number 2n = 18 host preference of winged asexual foundresses of the Grain aphid, oligonucleotide probes (Blackman & Eastop, 2000), clearly Sitobion avenae attracted to four main Poaceous hosts in a Latin- [(GATA)4]n, RAPDs cross breed. Using microsatellites, square arranged field experiment (36 aphid genotypes resolved (random amplified using RAPD molecular markers; hosts, from l. to r. = black Sunnucks et al. (1997) showed there (wheat); right-hand diagonal bars, (barley); horizontal bars polymorphic DNA; De to be high levels of allelic/genotype (cock’s-foot); left-hand diagonal bars (Yorkshire fog) (From: Barro et al., 1994; Lushai variation in Sitobion avenae sensu lato Lushai et al., 2002). et al., 1998) and collected from wheat and Dactylis loci is but a tiny proportion of the especially using AFLPs glomerata (cock’s-foot grass). There total genome and it is not known (amplified fragment length were found to be three apparently with any certainty what particular polymorphism; Forneck et al., almost non-interbreeding genotypic regions are stable and which are fast 2001a,b; Vorwerk & Forneck, 2007, groups, with high levels of sexual mutating, hence what the rest of the in press). Furthermore, such random recombination within each. Host genome is undergoing in terms of (somatic) mutations were seen within specialisation was apparent: thus mutational changes. 1-14 generations (S. avenae and there were wheat-specific lineages, Phylloxera), and in one case (S. Meanwhile, other regions are lineages common to both wheat and avenae) in the germ line (Lushai et seemingly changing within clonal D. glomerata, and lineages from D. al., 1998). Most extraordinary of all, lineages, including ribosomal DNA glomerata only which were found to Lushai et al. (1997) revealed using (rDNA) regions. Thus for example, bear many alleles from S. fragariae; RAPDs both intraclonal as well as Fenton et al. (1998a) showed that furthermore, the genotype class with intermorph banding pattern certain clonal Peach-potato aphid, S. avenae-like and S. fragariae-like differences in some S. avenae asexual Myzus persicae (Sulzer), lineages had alleles also carried S. fragariae-like lineages. As yet, the mechanism for two ITS (internal transcribed spacer) mtDNA in 80% of cases. Such such changes remains unknown, but haplotypes, suggestive of an asymmetry suggests that S. avenae may involve transposons, so called introgression event between M. males are attracted via ‘jumping genes’. persicae sensu stricto (s.s.) and another similar/identical sex pheromones close relative, M. certus (Walker). In terms of physiology, reference (Goldansaz, 2003) to S. fragariae Similarly, other clonal M. persicae has already been made to genetically- females (see Sunnucks et al., 1997 for lineages examined showed intra- and based intermorphic lineage further details and also Vialatte et al., inter-clonal polymorphism for the differences in colour and life cycle. 2005 ). number of IGS (intergenic spacer) Large variations have also been In R. padi, studies have shown that repeats (Fenton et al., 2003, 2005), recorded in the number of males the anholocyclic and holocyclic whilst persistent selection with produced between clones of different lineages are very divergent in terms of chemical pesticides (disulfoton) over life cycle forms of S. avenae, their mtDNA (by some 0.4 –1.4 200 generations (four years) has been especially androcyclic clones (Helden million years: Martinez-Torres, 1994 demonstrated to alter the IGS & Dixon, 2002). Lastly, highly cited in Simon et al., 1996). Newer genotype of Greenbug, Schizaphis insecticide-resistant M. persicae clones evidence suggests that a hybridisation graminum (Rondani) clones, involving (R2 and R3) which show event may have occurred in recent the loss of specific bands (Shufran et amplification of the esterase-4 (E4) historical times between R. padi s.s. al., 2003). Blackman et al. (2000), gene (which confers resistance and to and a closely-related unknown using fluorescent in-situ hybridisation some degree cross-resistance in this species, causing asexuality (Delmotte (FISH) techniques, showed species), are known to undergo a et al., 2003). Other evidence points Antenna 31 (4) 195 to there being ‘sexual leakage’ (gene avenae infesting wheat, wheat possible but unlikely event (Hille Ris flow) between lifecycle morphs and volunteers, barley and maize was also Lambers, 1939; Dicker, 1940; that asexuals derive from sexuals, found in French populations using Blackman & Eastop, 2000; see also probably by three main mechanisms, microsatellites (Haack et al. 2000). Tatchell et al., 1983). including hybridisation (Delmotte et Thus many genotypes were detected, The available evidence reveals al., 2001). As mentioned earlier, some with apparent host preferences, convincingly that a ‘Janzenian’ super- whilst microsatellites reveal multi- whilst two genotypes from maize clonal population is unlikely to prove locus genotypes to be identical at a were also found on all the other a reality, because of the clear range of loci (Haack et al., 2000), the hosts. The data suggest specialist (s) molecular heterogeneity and levels of rest of the genome is untested and and generalist (g) clones, ‘g’ clones host adaptation shown to exist within may conceal widespread variations, seemingly being able to colonise large and between natural aphid both within and between asexual geographical areas and persist for populations of given species (real or lineages. Hence at the present time it several years. Such a scenario could apparent). Even so, as shown using is not possible to say definitively that be favoured by agricultural practice. molecular genetic markers, some a given aphid species asexual lineage Besides these two ‘g’ clones, a generalist clones of certain species has identity in any strict genetic sense, continual replacement of rare ‘s’ (so-called ‘Superclones’) have a wide without, that is, sequencing the entire genotypes was observed in maize in distribution and may persist for some nuclear and mitochondrial genomes both years of the study.