Transmission of Two Viruses that Cause Barley Yellow Dwarf is Controlled by Different Loci in the Aphid, Schizaphis graminum

Clonal populations of the aphid, Schizaphis graminum, have been separated into biotypes based on host preference and their ability to overcome resistance genes in wheat. Recently, several biotypes were found to differ in their ability to transmit one or more of the viruses that cause barley yellow dwarf disease in grain crops, and vector competence was linked to host preference. The genetics of host preference has been studied in S. graminum, but how this may relate to the transmission of plant viruses is unknown. Sexual morphs of a vector and nonvector S. graminum genotype were induced from parthenogenetic females and reciprocal crosses made. Eighty-nine hybrids were generated and maintained by parthenogenesis. Each hybrid was evaluated for its ability to transmit Barley yellow dwarf virus-PAV and Cereal yellow dwarf virus-RPV, and for its ability to colonize two wheat genotypes each expressing a different gene that confers resistance to S. graminum. The F1 genotypes were genetically variable for their ability to transmit virus and to colonize the aphid resistant wheat, but these traits were not genetically correlated. Individual F1 genotypes ranged in transmission efficiency from 0–100% for both viruses, although the overall mean transmission efficiency was similar to the transmission competent parent, indicating directional dominance. The direction of the cross did not significantly affect the vector competency for either virus, suggesting that maternally inherited cytoplasmic factors, or bacterial endosymbionts, did not contribute significantly to the inheritance of vector competency in S. graminum. Importantly, there was no genetic correlation between the ability to transmit Barley yellow dwarf virus and Cereal yellow dwarf virus-RPV in the F1 genotypes. These results taken together indicate that multiple loci are involved in the circulative transmission, and that the successful transmission of these closely related viruses is regulated by different sets of aphid genes

Inbreeding Avoidance by Recognition of Close Kin in the Pea Aphid, Acyrthosiphon pisum

Inbreeding depression has detrimental effects on many organisms, but its effects are potentially greater in organisms that have at least one asexually reproducing life stage. Here, the existence of severe inbreeding depression upon selfing (r = 1) in the cyclic parthenogenetic aphid Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae) is documented. Egg hatching success and offspring survival of inbred mating pairs are significantly lower than that of outbred mating pairs. Two possible mechanisms for avoiding selfing are examined: avoidance of partners of identical genetic makeup and avoidance of partners of the same body color (as a proxy for genetic similarity). Mating between males and females of the same color was as successful as mating between partners of different colors. In contrast, the success of mating between close kin was consistently reduced compared to that of mating between genetically unrelated partners. Interestingly, mating between close kin proceeded normally until the very last stage of the mating process. Thus, inbreeding avoidance appears to take place sometime between copulation and sperm transfer, suggesting that cryptic female choice may play a role in the process. © This is an open access paper. We use the Creative Commons Attribution 3.0 license that permits unrestricted use, provided that the paper is properly attributed

Genetics of Color Polymorphism in the Pea Aphid, Acyrthosiphon pisum

The genetic basis of color polymorphism is explored in the pea aphid, Acyrthosiphon pisum (Harris) (Homoptera: Sternorrhyncha), in which two color morphs have been described (pink or green). Laboratory crosses and a Mendelian genetic analysis reveal that color polymorphism in pea aphids is determined by a single biallelic locus, which we name colorama, with alleles P and p, pink being dominant to green. The putative genotypes are Pp or PP for pink morphs, and pp for green morphs. This locus is shown to be autosomal. Last, there was no evidence of influence of the direction of the cross on color inheritance, thus showing that cytoplasmic effects and/or maternally-inherited symbionts play no role in the inheritance of color polymorphism in pea aphids. The existence of a simple genetic determinism for color polymorphism in a system in which genetic investigation is possible may facilitate investigations on the physiological and molecular mechanisms of genetically-based color morph variation, and the establishment of a link between this locus and fitness in a range of ecological conditions

Recognition of host-specific chemical stimulants in two sympatric host races of the pea aphid Acyrthosiphon pisum

1. In ecological speciation, adaptation to variation in the external environment provides the crucial push that starts the process of genetic divergence and eventually leads to speciation. This emphasis on the role of ecological specialisation in speciation events has brought with it a renewed interest in its proximate mechanisms in recently diverged groups such as host races. Here, the proximate mechanisms of feeding specialisation are investigated in two host races of the pea aphid Acyrthosiphon pisum. 2. Using alfalfa and clover extracts, enclosed in diet chambers or applied on whole plants, it is shown that feeding specialisation depends on recognition of stimulants specific to the host plant, not on deterrents or toxins specific to the non-host plants. 3. Because pea aphids mate on their host plant, feeding specialisation leads to de facto assortative mating. This study suggests that behavioural recognition of host-specific chemicals, rather than avoidance of deterrents or/and plant toxins, contributes to gene flow restriction between the alfalfa and clover host races