ELECTROPHORETIC STUDY OF 5 APHID PARASITOID SPECIES OF THE GENUS APHIDIUS (HYMENOPTERA, BRACONIDAE), INCLUDING EVIDENCE FOR REPRODUCTIVELY ISOLATED SYMPATRIC POPULATIONS AND A CRYPTIC SPECIES

Four polymorphic enzymes (PEP, PGI, PGM and IDH) were separated from a dult individuals of five aphid parasitoid species of the genus Aphidiu s Nees (A. ervi Haliday, A. microlophii Pennacchio & Tremblay, A. eady i Stary, Gonzalez & Hall, A. picipes Nees and A. urticae Haliday) usin g horizontal cellulose acetate plate electrophoresis. These markers we re used to investigate the genetic relationships, including reproducti ve isolation and host adaptation/specificity, in laboratory and field populations. Samples were collected from the pea aphid, Acyrthosiphon pisum (Harris) and/or the nettle aphid, Microlophium carnosum (Buckton ) in the UK and Bulgaria between 1991 and 1994. Whilst all loci discri minated between some species, PGM discriminated all five species, one species (A. eadyi) bearing two unique alleles (PGM(a) and PGM(c)). Aph idius microlophii (from nettle aphid) and A. ervi (from pea aphid), wh ich are difficult to separate morphologically, possessed unique PGM al leles - PGM(b) and PGM(e), respectively. Both parasitoids occur sympat rically, and whilst hybrids heterozygous for PGM were produced in the laboratory (pGM(b,e)), such genotypes were not observed in the field p opulations sampled. Hence, the species appear to be reproductively iso lated. Most parasitoid populations studied showed mean heterozygote de ficiencies per locus (homozygote excess) compared with Hardy-Weinberg expectations. In particular, A. eadyi bearing PGM(a) alleles were alwa ys homozygous whilst additionally, many were homozygous for another al lele, PGI(b). This is evidence for the existence of one or more morpho logically-indistinguishable 'cryptic' species occurring sympatrically within European field populations. A dendrogram of relatedness was pro duced following calculation of Nei's genetic identity coefficient, I f rom the parasitoid population allele frequency data. All species showe d very high similarity between populations at the intraspecific level (>0.9), but fewer interspecific similarities (0.23-0.63). These values compare well with previously published values for Aphidius population s and for other insects.