Dinah F. Hales

Genetic structure of an aphid studied using microsatellites: cyclic parthenogenesis, differentiated lineages and host specialization

In a previous study, samples of the grain aphid Sitobion avenae (F.) were collected from wheat and adjacent cocksfoot hosts in a population thought to be primarily parthenogenetic, and DNA from individual aphids was analysed with a multilocus technique. Here we have applied single‐locus microsatellites and a mitochondrial DNA marker to a subset of the same DNA extracts, and have made several additional inferences about important genetic and population processes in S. avenae. Microsatellite analysis indicated very high levels of genic and genotypic variation. S. avenae fell into three genotypic groups inferred to be almost noninterbreeding, while analysis of linkage and Hardy‐Weinberg equilibria suggested high levels of sexual recombination within each genotypic group. Host specialization was evident: one lineage was found only on wheat, and one (bearing many alleles inferred to be introgressed from the blackberry‐grass aphid S. fragariae (Walker)) was found only on cocksfoot. The third group of interrelated genotypes was found commonly on both hosts. Although most genotypes were found only once, some were much more numerous in the sample than expected from the frequency of the alleles they contained. This, and rapid temporal changes in genotypic composition of samples, indicates strong selective differences between genotypes and lineages. In the major genotypic group, the commonest genotypes were significantly more homozygous than were rare ones: thus these data may help to explain the frequent observation of homozygous excess in aphid allozymes. The genotype group showing S. avenae‐like as well as S. fragariae‐like alleles also carried S. fragariae‐like mitochondrial DNA in at least 25/31 cases, indicating gender‐asymmetrical hybridization.

Microevolution, low clonal diversity and genetic affinities of parthenogenetic Sitobion aphids in New Zealand

In sharp contrast to their southeast Asian and European counterparts, Sitobion miscanthi and S. near fragariae aphids in Australia exhibit a complete absence of sexual reproduction. This demands an explanation within the context of the evolution and maintenance of sex and parthenogenesis. Accordingly, we executed a genetic analysis of the two species in neighbouring New Zealand. Microsatellites and single‐stranded conformation polymorphism/sequence analysis of the nuclear gene elongation factor 1α were used to identify aphid clones and confirm species identification, respectively. Karyotypic variation was also investigated. The New Zealand fauna showed few (nonrecombining) genotypes and appears to have received migrants from both Australia and Asia. Other genotypes have apparently arisen in situ in New Zealand, exhibiting stepwise mutation of microsatellite alleles and also karyotypic change. Thus, these data represent rare evidence of evolution within wild‐living parthenogenetic lineages. Karyotypic changes appear to occur at a rate even greater than that of microsatellite evolution. Strong geographical partitioning of genotypes/karyotypes was found, with certain ones predominating over large areas. These data suggest that clonal selection could be important in the distribution and patterning of genetic variation. We present a model to explain the genetic patterns, with particular reference to the absence of sexual reproduction in Sitobion aphids in New Zealand and Australia.

Microsatellite isolation, linkage group identification and determination of recombination frequency in the peach-potato aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae)

Fifteen polymorphic microsatellite markers were used to establish linkage groups and relative rates of recombination in male and female Myzus persicae (Sulzer) (Hemiptera: Aphididae) (peach-potato aphid). We cloned nine markers from M. persicae and for these we report primer sequences and levels of allelic diversity and heterozygosity in four Australian M. persicae populations. Of the remaining six loci, four loci, also cloned from M. persicae, were obtained from G. Malarky (Natural History Museum, London) and two loci from Sitobion miscanthi were used. Additionally, the primer sequences of locus M77, a locus monomorphic in M. persicae but polymorphic in the closely related Myzus antirrhinii, are presented. Eleven of the 15 polymorphic markers were autosomal and four were X-linked. A linkage analysis was performed on a European pedigree of aphids containing five families with between seven and 11 offspring each. There was no linkage between any loci in females. In males, several pairwise comparisons yielded no recombinant offspring. With the exception of locus M40, these observations were supported in a linkage analysis performed on larger families produced from Australian M. persicae crosses. Locus M40 showed segregation consistent with involvement in a translocation between autosomes 1 and 3 in European samples but not in the Australian samples. From the Australian crosses we report an absence of recombination in males but high recombination rates in females. One X chromosome and four autosomal linkage groups were identified and tentatively assigned to chromosomes. The relevance of achiasmate meiosis to the evolution of sex is discussed.

Random loss of X chromosome at male determination in an aphid, Sitobion near fragariae, detected using an X-linked polymorphic microsatellite marker

This paper reports the first direct molecular evidence that X chromosome loss during determination of male aphids (XO) is random. Clonal and sexual females, and males, of the species Sitobion near fragariae were screened using three polymorphic microsatellite loci. Two loci, Sm10 and Sm17, showed the same heterozygous genotypes in all three aphid morphs. The third, Sm11, was heterozygous for the same two alleles in clonal and sexual females, but of the 25 males screened 11 showed the ‘160’ allele and 14 showed the ‘156’ allele. This result indicates X-linkage of locus Sm11, with random loss of the X chromosome during the formation of male embryos. The possible implications of this result are discussed with respect to aphid sex determination, recombination and chromosome evolution.

Evolution of an ecological trait in parthenogenetic Sitobion aphids.

The aim of this study was to test whether host plant responses of Sitobion aphids have evolved under parthenogenesis and to examine the relationship between genetic and phenotypical similarity. There are four known chromosomal races of Australian Sitobion miscanthi living on grasses. Three races evolved from a recent common ancestor by mutation and chromosomal rearrangement alone. Australian S. miscanthi reproduce entirely by parthenogenesis, as does the close congeneric S. near fragariae. Mean relative growth rate (MRGR) was investigated in laboratory-raised representative aphid clones of four races of S. miscanthi, and also S. near fragariae (i.e. five aphid ‘forms’) on three host plants, with 15 replicate aphids per clone. There were significant differences in MRGR; most variance was associated with differences among forms, some among clones within forms and very little with aphids within clones. Developmental time and adult weight both contributed to the differences in MRGR. There was a significant interaction for clone(nested within forms)×host for all three dependent variables. No one clone performed significantly better over all hosts than other clones of its form (clonal MRGRs on the three hosts were negatively correlated). Nearly all clones performed best on barley (which was the only ‘familiar’ host, in that previous generations had been raised on it), next best on cocksfoot and worst on rye. MRGR was found to be under genetic control. The data show that monophyletic parthenogenetic aphids can rapidly evolve substantial differences in host relations and suggest a possible association of chromosomal rearrangements with MRGR.

Behaviour of the X chromosomes during growth and maturation of parthenogenetic eggs of Amphorophora tuberculata (Homoptera, Aphididae), in relation to sex determination

Prophase chromosomes of growing oocytes from thelytokous, viviparous females of Amphorophora tuberculata Brown and Blackman (n=2) were studied using a modified propionic acid squash technique with Feulgen staining. In early prophase, prior to the growth phase of the oocyte, the X chromosomes are partially condensed and looped together so that all four ends appear to be associated. Later in prophase the X chromosomes separate in oocytes destined to be female, but remain associated in presumptive male oocytes. The autosomes condense gradually throughout prophase. The nucleus of the presumptive male oocyte is further characterised by the formation of a spherical Feulgen-positive body, which attains a large size (7 μm diameter) in late prophase. At this stage, the X chromosomes are no longer visible as separate entities, and are apparently included in the spherical body. At metaphase this disappears, leaving the X chromosomes still united as a condensed bivalent. The spherical body seems to have nucleolar as well as chromatin constituents; nucleolar organisers are present at the ends of the X chromosomes where it first arises. It may function in maintaining the cohesion between the X chromosomes through prophase, and could also facilitate correct orientation of the X bivalent on the spindle of the maturation division. As sex determination in aphids is controlled by juvenile hormone concentration, it appears that the hormone may interact with the X chromosomes during prophase, bringing about their separation in female oocytes, perhaps by inhibiting the formation of the spherical body.

Lack of detectable genetic recombination on the X chromosome during the parthenogenetic production of female and male aphids

We used polymorphic microsatellite markers to look for recombination during parthenogenetic oogenesis between the X chromosomes of aphids of the tribe Macrosiphini. We examined the X chromosome because it comprises approximately 25 % of the genome and previous cytological observations of chromosome pairing and nucleolar organizer (NOR) heteromorphism suggest recombination, although the same is not true for autosomes. A total of 564 parthenogenetic females of Myzus clones with three distinct reproductive modes (cyclical parthenogenesis, obligate parthenogenesis and obligate parthenogenesis with male production) were genotyped at three informative X-linked loci. Also, parthenogenetically produced males from clones encompassing the full range of male-producing reproductive strategies were genotyped. These included 391 Myzus persicae males that were genotyped at three X-linked loci and 538 males from Sitobion clones that were genotyped at five informative X-linked loci. Our results show no departure from clonality in parthenogenetic generations of aphids of the tribe Macrosiphini: no recombinant genotypes were observed in parthenogenetically produced males or females.

The chromosomes of Schoutedenia lutea (homoptera, aphidoidea, greenideinae), with an account of meiosis in the male

Somatic chromosomes of both sexes and chromosome behaviour during spermatogenesis were studied in the aphid Schoutedenia lutea (van der Goot). Four long but unequal chromosomes in females were interpreted as X chromosomes (X1X1X2X2) with one member of an autosome pair attached to one X1, and the other member to one X2, so that the four long chromosomes were actually X1+A, X1, X2+A, X2. Males (normally XO in aphids) received X chromosomes corresponding in relative length to the two longest (X1+A, X2+A) in females. During spermatogenesis parallel pairing occurred in prophase 1 and the X1 and X2 chromosomes became associated via their autosomal segments. In anaphase I, the autosomal segment became detached from one of the X chromosomes and entered the non-viable (non-X-bearing) spermatocyte, while the viable spermatocyte received both X1 and X2 (either one of which still carried an autosome) and the haploid set of free autosomes. The consequences for sex determination and zygote formation of this unusual system are discussed; a stable chromosomal constitution for the zygote can be achieved only at the expense of considerable gamete wastage.

JUVENILE HORMONE AND APHID POLYMORPHISM

ABSTRACT Evidence for the role of juvenile hormone in determination of apterous parthenogenetic aphids is reviewed. Corpus allatum activity, assessed by volume or nuclear size, is greater in adult aphids producing apterous young, and in larval aphids which are developing as apterae. On the basis of these relationships, a consistent hypothesis for the control of production of apterae by juvenile hormone can be proposed. However, experiments with juvenile hormone analogues have given equivocal or negative results and the hypothesis remains unconfirmed. Some of the difficulties involved in experimental work of this kind are indicated.

Precocene causes male determination in the aphid Myzus persicae

When adult apterous viviparous females of Myzus persicae, reared in short night conditions at 21–23°C, are treated with the precocene analogue 6-methoxy-7-ethoxy-2,2-dimethylchromene, they deposit males towards the end of their reproductive lives. The first-born normal daughters of the treated females also deposit some males at various times in their reproductive lives. Karyotypic analysis was used to investigate the sequence of male and female embryos in the ovarioles of precociously metamorphosed aphids. The experiments support the hypothesis (Mittler et al., 1979) that juvenile hormone level controls the sex determination process in aphids. Since male aphids have an XO sex chromosome constitution, this implies that juvenile hormone level influences the behaviour of the X-chromosomes at or before the single maturation division of the egg. At this division one X-chromosome is eliminated from eggs which will develop as males. Aphids provide the first example of a specific endocrine influence on chromosome behaviour in sex determination.