Date of Award:

5-1-1998

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Biology

Committee Chair(s)

Carol D. von Dohlen

Committee

Carol D. von Dohlen

Committee

Paul G. Wolf

Committee

Nabil N. Youssef

Abstract

Most sternorrhynchan insects harbor endosymbiotic bacteria in specialized cells (bacteriocytes) near the gut, which provide essential nutrients for hosts. In lineages investigated so far with molecular methods (aphids, mealybugs, whiteflies), endosymbionts apparently have arisen from independent infections of common host ancestors and cospeciated with their hosts. Some endosymbionts also exhibit putatively negative genetic effects from their symbiotic association. In this study, the identity of endosymbionts in one major sternorrhynchan lineage, psyllids (Psylloidea), was investigated to determine their position in eubacterial phylogeny and relationship to other sternorrhynchan endosymbionts. Small-subunit ribosomal RNA genes (16S rDNA) from bacteria in three psyllid species (families Psyllidae and Triozidae) were sequenced and incorporated into an alignment including other insect endosymbionts and free-living bacteria. In phylogenetic analysis, all sequences were placed within the y-subdivision of the Proteobacteria. Three sequences, one from each psyllid species, formed a highly supported monophyletic group whose branching order matched the host phylogeny, and also exhibited accelerated rates of evolution and mutational bias toward A and T nucleotides. These attributes, characteristic of primary (P), bacteriocyte-dwelling endosymbionts, suggested that these sequences were from the putative psyllid P endosymbiont. Two other sequences were placed within the y-3 subgroup of Proteobacteria and were hypothesized to be secondary endosymbionts. The analysis also suggested a sister relationship between P endosymbionts of psyllids and whiteflies. Thus, a continuous, mutualistic association between bacteria and insects may have existed since the common ancestor of psyllids and whiteflies. Calculations using a universal substitution rate in bacteria corrected for endosymbiont rate acceleration support the idea that this common ancestor was also the ancestor of all Sternorrhyncha. Compared to other P endosymbiont lineages, the genetic consequences of intracellular life for some psyllid endosymbionts have become exaggerated, indicating possible differences in population structure of bacteria and/or hosts.

Download

Included in

Biology Commons

Share

COinS

Copyright for this work is retained by the student. If you have any questions regarding the inclusion of this work in the Digital Commons, please email us at .