Biology Reference
In-Depth Information
2
Buchnera
Bacteria and Other
Symbionts of Aphids
Angela E. Douglas
CONTENTS
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Buchnera
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Buchnera
A HISTORICAL OVERVIEW OF SYMBIOTIC BACTERIA IN APHIDS
The bacterial symbiosis in aphids was Ýrst detected, but not recognized, by the eminent 19th-
century evolutionary biologist. T. H. Huxley. Huxley noticed cells with Ñgranular protoplasmÒ in
the body cavity of aphids. He mistakenly identiÝed these cells as accessory fat body, which he
called the Ñpseudovitellus,Ò and the granular inclusions as Ñvitelline spheres.Ò But perhaps this error
should not affect HuxleyÔs wider reputation, for, as HuxleyÔs biographer Adrian Desmond writes,
Ñaphids might not amount to much on the cosmic scaleÒ (Desmond, 1997)!
The true nature of the granular protoplasm in the aphid ÑpseudovitellusÒ Ð bacteria tightly
packed into the cytoplasm of insect cells Ð was Ýrst appreciated by Sulc and Pierantoni in 1910
(Pierantoni, 1910; Sulc, 1910). Microscopic analyses conducted during the Ýrst half of the last
century (summarized in Buchner, 1966) revealed that virtually all aphids apart from the Phyllox-
eridae and Adelgidae bore coccoid bacteria in large cells, termed mycetocytes. Furthermore, these
bacteria were transmitted from mother aphid to offspring via the maternal ovaries, a process called
transovarial transmission, such that each egg produced by sexual oviparous females and each larva
deposited by asexual parthenogenetic females bore bacteria derived from the mother. Some, but
not all, aphid species also bore rod-shaped bacteria, readily distinguishable from the coccoid bacteria
and transmitted transovarially with the cocci. Buchner (1966) termed the universal cocci Ñprimary
symbiontsÒ and the other bacteria Ñaccessory symbiontsÒ; the latter are also often known as
Ñsecondary symbionts.Ò
The remarkable bacterial associations in aphids were deemed intractable to study. In particular,
all attempts to isolate the primary symbionts into axenic culture failed; consequently, the bacteria
