Clonal mixing in the soldier-producing aphid Pemphigus spyrothecae 1 (Hemiptera: Aphididae) 2 3

Abstract 1 Illuminating the genetic relationships within soldier-producing aphid colonies is an 2 essential element of any attempt to explain the evolution of the altruistic soldier caste. 3 Pemphigus spyrothecae is a soldier-producing aphid that induces galls on the leaf 4 petioles of its host (trees of the genus Populus). At least a quarter of the aphids within 5 the clonally-produced gall population are morphologically and behaviourally distinct 6 first-instar soldiers that defend the gall population from predation. Using field 7 trapping and microsatellites, we investigated the degree of clonal mixing within 8 natural gall populations. Field trapping in the UK showed that all the migrants of P. 9 spyrothecae and of two other Pemphigus species were wingless first-instar soldiers. 10 The average degree of mixing estimated from trapping P. spyrothecae migrants was 11 0.68% (range = 0-15%). Microsatellite genotyping of 277 aphids from thirteen galls 12 collected in Italy revealed an average mixing level of 10.4% (range = 0-59%). Six 13 galls contained more than one clone (range = 2-5). Non-kin aphids were not restricted 14 to the soldier caste but were evenly distributed across instars. An additional gall, from 15 which 527 occupants were genotyped, contained 12 non-kin aphids distributed among 16 9 clones, showing that clonal diversity can be high even when mixing is very low. 17 These observations suggest that although soldiers migrate regularly and can moult and 18 reproduce within foreign galls, clonal mixing in this species is generally low and is 19 unlikely to provide a barrier to the evolution of investment by the aphid clones in an 20 altruistic soldier caste