HOST-CELL ALLOMETRY AND REGULATION OF THE SYMBIOSIS BETWEEN PEA APHIDS, ACYRTHOSIPHON-PISUM, AND BACTERIA, BUCHNERA

The symbiotic bacteria Buchnera in aphids are borne in cells, called b acteriocytes, in the insect haemocoel. The number and median volume of bacteriocytes in pre-reproductive adult insects varied significantly among 14 parthenogenetic clones of the pea aphid Acyrthosiphon pisum. After logarithmic transformation of the data, the relationship of both number and median volume of bacteriocytes with aphid weight for the c lones could be described by common regression lines with slopes signif icantly greater than zero. The allometric slope for median bacteriocyt e volume was calculated as 1.06, by model I regression and 1.94 by mod el II regression; and the equivalent values of the allometric slope fo r total volume of bacteriocytes were 1.51 and 2.50, suggesting that th e total volume of bacteriocytes increases disproportionately with aphi d body weight. The partial correlation coefficient between the number and median volume of bacteriocytes was + 0.07, with body weight held c onstant. It is proposed that the regulation of number and size of bact eriocytes is not linked and that bacteriocytes may not exhibit compens atory changes in size, in response to alteration in number. Experiment al manipulation of-the rates of bacteriocyte differentiation and divis ion could therefore perturb the total volume of the symbiosis, on whic h aphid pests depend for normal growth and reproduction. (C) 1998 Else vier Science Ltd. All rights reserved.