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The mechanism of obligate endosymbionts limiting thermal toler-ance of host species was revealed

 

Recently, Dr. Zhang Bo from the Innovation Team for Protection and Application of Insect Natural Enemies of IPP-CAAS published a paper entitled “Obligate bacterial endosymbionts limit thermal tolerance of insect host species” on Proceedings of the National Academy of Sciences of the United States of America (PNAS), confirming that the heat sensitivity of aphid endosymbionts contribute to host variation in heat tolerance.

Thermal tolerance is a basic determinant of an organism’s ecology: the ability to sur-vive and to reproduce in the face of variation in temperature affects geographic range, competitive ability, and population dynamics. Many animals and plants en-gage in intimate symbiotic interactions with microorganisms, and these symbionts can impact thermal tolerance. When hosts are dependent on symbionts that are themselves heat-sensitive or cold-sensitive, the host’s temperature range and geo-graphic distribution may be curtailed by the symbiosis. Aphids and their intracellu-lar bacterial associates, Buchnera aphidicola, are a central widely studied model of obligate symbiosis. Buchnera has diversified with aphids through maternal transmis-sion for >100 million years; their tiny genomes encode less than 600 proteins but re-tain genes underlying production of amino acids needed for host nutrition. Several observations suggest that Buchnera is heat-sensitive. The hypothesis is whether aphid heat tolerance is determined by Buchnera limits the heat tolerance of aphid hosts, and whether ’s tolerance and whether variation among aphid species in heat tolerance is linked to variation in Buchnera’s tolerance, because aphid species vary widely in heat sensitivity.

This study documented aphid survivorship and reproduction following heat chal-lenge in five widely distributed aphid species chosen on the basis of apparent differ-ences in thermal tolerance, partly reflected in geographic ranges. Aphis craccivoraand Aphis gossypii are generalist crop pests distributed worldwide in warm temperate and tropical regions, whereas Aphis fabae is a generalist that is most common in cool regions, including northern Europe and Canada. Acyrthosi-phon kondoi and Ac. pisum spe-cialize on alfalfa and favor cool environments. The authors included a second Ac. pi-sum line in which Buchnera bears a single nucleotide deletion in the ibpA heat shock promoter, increasing heat sensitivity. They also quantified symbiont titers fol-lowing heat exposure, visualized bacteriocytes with and without heat exposure, and explored the genomic and transcriptional underpinnings of heat responses in both aphid host and Buchnera. Fecundity following heat exposure is severely decreased by a Buchnera mutation that suppresses transcriptional response of a gene encoding a small heat shock protein. Similarly, absence of this Buchnera heat shock gene may explain the heat sensitivity of Ap. fabae. Fluorescent in situ hybridization revealed that heat-induced deformation and shrinkage of bacteriocytes in heat-sensitive spe-cies, but not in heat-tolerant species. Sensitive and tolerant species also differed in numbers and transcriptional responses of heat shock genes. While heat-induced re-sponses of aphid hosts themselves also may contribute, symbiont ability to withstand heat appears to be a central determinant of host thermal range.

This paper was resulted from the collaboration between Assoc. Prof. Zhang Bo at IPP-CAAS and Dr. Nancy A. Moran’s research group at University of Texas-Austin. And the work was supported by the China Scholarship Council and the Science and Tech-nology Innovation Program of CAAS.

By Zhang Bo (Zhangbo05@caas.cn)




Copyright:Institute of Plant Protection,Chinese Academy of Agricultural Sciences 

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