Symbiotic system of the Japanese common plataspid stinkbug Megacopta punctatissima

The plataspid stinkbug, Megacopta punctatissima (Figure 5 . 1A), is commonly found everywhere in mainland Japan, feeding mainly on the leguminous vine, Pueraria lobata, although

Figure 5.3 (Color figure follows p. 238.) (A) Egg masses of Megacopta punctatissima laid on a bud of Pueraria lobata. (B) Symbiont capsules, dark brown in color, placed on the underside of an egg mass . (C) An isolated egg and capsules . (D) A newborn nymph probing a symbiont capsule. Bars show 0 . 5 mm . (From Fukatsu, T., and Hosokawa, T. [2002] . Appl. Environ. Microbiol. 68: 389-396. With permission )

Figure 5.3 (Color figure follows p. 238.) (A) Egg masses of Megacopta punctatissima laid on a bud of Pueraria lobata. (B) Symbiont capsules, dark brown in color, placed on the underside of an egg mass . (C) An isolated egg and capsules . (D) A newborn nymph probing a symbiont capsule. Bars show 0 . 5 mm . (From Fukatsu, T., and Hosokawa, T. [2002] . Appl. Environ. Microbiol. 68: 389-396. With permission )

other leguminous plants, including crop legumes like soybean and pea, are occasionally utilized (Tomokuni, 1993). From May to July, adult females lay egg masses on buds of P. lobata An egg mass consists of 5 to 51 eggs arranged in two rows in parallel (Figure 5 3A) On the underside of the egg masses, symbiont capsules, 0 2 to 0 3 mm in diameter and dark brown in color, are always attached (Figure 5 .3B) . The aubergine-shaped eggs and the capsules are easily separable with forceps under a dissection microscope (Figure 5 3C) Within a week after oviposition, first instar nymphs emerge from the eggs, and the newborn nymphs immediately probe the capsules with their proboscis for 1 hour or so (Figure 5 3D) After sucking the capsule, they enter a resting period in aggregation nearby the eggshells for 1 or 2 days, and then disperse for feeding on the plant sap

Figure 5 .4 shows the internal structure of the symbiont capsule . Light microscopy indicated that the capsule consists of two distinct components, namely, a layered envelope and the capsule content (Figure 5 4A) Careful dissection of fresh capsules revealed the cuticlelike envelope and the resin-like content, both of which are brownish in color Electron microscopy unveiled that the envelope encases not only bacterial cells but also a matrix

Figure 5.4 (Color figure follows p. 238.) Light and electron microscopy of sectioned symbiont capsules of Megacopta punctatissima. (A) Light microscopic image of a symbiont capsule . (B) Electron microscopic image of a symbiont capsule . Symbiont cells are shown by asterisks . (C) Electron microscopic image of the capsule content . Abbreviations: CO, capsule content; LE, layered envelope; SM, secretion matrix . Bars show 2 ¡urn in (A) and (B), and 1 ¡um in (C) . (From Hosokawa, T., Kikuchi, Y., Meng, X .Y., and Fukatsu, T. [2005] . FEMS Microbiol. Ecol. 54: 471-477. With permission .)

Figure 5.4 (Color figure follows p. 238.) Light and electron microscopy of sectioned symbiont capsules of Megacopta punctatissima. (A) Light microscopic image of a symbiont capsule . (B) Electron microscopic image of a symbiont capsule . Symbiont cells are shown by asterisks . (C) Electron microscopic image of the capsule content . Abbreviations: CO, capsule content; LE, layered envelope; SM, secretion matrix . Bars show 2 ¡urn in (A) and (B), and 1 ¡um in (C) . (From Hosokawa, T., Kikuchi, Y., Meng, X .Y., and Fukatsu, T. [2005] . FEMS Microbiol. Ecol. 54: 471-477. With permission .)

Figure 5.5 (Color figure follows p. 238.) Dissected posterior midgut from adult female (A) and adult male (B) of Megacopta punctatissima. Abbreviations: AEM, anterior enlarged midgut section; BEM, brownish enlarged midgut end section; MPT, Malpighian tubules; REC, rectum; SCM, swollen crypt-bearing midgut section; TCM, thin crypt-bearing midgut section . Bars show 1 mm . (From Hosokawa, T., Kikuchi, Y., Meng, X.Y., and Fukatsu T. [2005] . FEMS Microbiol. Ecol. 54: 471-477. With permission )

Figure 5.5 (Color figure follows p. 238.) Dissected posterior midgut from adult female (A) and adult male (B) of Megacopta punctatissima. Abbreviations: AEM, anterior enlarged midgut section; BEM, brownish enlarged midgut end section; MPT, Malpighian tubules; REC, rectum; SCM, swollen crypt-bearing midgut section; TCM, thin crypt-bearing midgut section . Bars show 1 mm . (From Hosokawa, T., Kikuchi, Y., Meng, X.Y., and Fukatsu T. [2005] . FEMS Microbiol. Ecol. 54: 471-477. With permission )

(Figures 5 .4B and C) . The symbiont cells were buried in the matrix rather sparsely, accounting for less than half of the volume of the capsule content (Figure 5 .4C) .

Figure 5 5 shows the posterior midgut dissected from adult insects of M. punctatissima In females, anatomically distinct sections were recognized along the alimentary tract: an anterior enlarged midgut (AEM) section, a thin crypt-bearing midgut (TCM) section, a swollen crypt-bearing midgut (SCM) section, and a brownish enlarged midgut (BEM) end section (Figure 5 5A) In males, no such specializations were found: nearly the full length of the alimentary tract was represented by a TCM section while SCM and BEM sections were lacking (Figure 5 5B)

Figure 5 . 6 shows the light and electron microscopy of the female alimentary tract, which unveiled functional specialization of each of the midgut sections The TCM section, accounting for the largest part of the posterior midgut, bore a number of voluminous crypts (Figure 5 . 6A) . The crypts were lined with very thin epithelial cells and filled with

Figure 5.6 (Color figure follows p. 238.) Light and electron microscopy of the midgut sections of Megacopta punctatissima. (A) Thin crypt-bearing midgut section (TCM) . (B) Crypts of TCM, where numerous symbiont cells (asterisks) and thin epithelium are seen . (C) Swollen crypt-bearing midgut section (SCM) . (D) Crypt of SCM, where the matrix is secreted . In the main tract of the midgut, a number of symbiont cells (asterisks) are embedded in the matrix . (E) Brownish enlarged midgut end section (BEM) . (F) Crypts of BEM, whose cavity is filled with filament-like materials of the capsule envelope . Abbreviations: EP, epithelium; FM, filament-like material; GC, gut content . Bars show 50 ¡um in (A), (C), and (E), and 2 ¡urn in (B), (D), and (F) . (From Hosokawa, T., Kikuchi, Y., Meng, X .Y., and Fukatsu T. [2005] . FEMS Microbiol. Ecol. 54: 471-477. With permission .)

Figure 5.6 (Color figure follows p. 238.) Light and electron microscopy of the midgut sections of Megacopta punctatissima. (A) Thin crypt-bearing midgut section (TCM) . (B) Crypts of TCM, where numerous symbiont cells (asterisks) and thin epithelium are seen . (C) Swollen crypt-bearing midgut section (SCM) . (D) Crypt of SCM, where the matrix is secreted . In the main tract of the midgut, a number of symbiont cells (asterisks) are embedded in the matrix . (E) Brownish enlarged midgut end section (BEM) . (F) Crypts of BEM, whose cavity is filled with filament-like materials of the capsule envelope . Abbreviations: EP, epithelium; FM, filament-like material; GC, gut content . Bars show 50 ¡um in (A), (C), and (E), and 2 ¡urn in (B), (D), and (F) . (From Hosokawa, T., Kikuchi, Y., Meng, X .Y., and Fukatsu T. [2005] . FEMS Microbiol. Ecol. 54: 471-477. With permission .)

symbiotic bacteria . The symbionts were tightly packed in the cavity of the crypts, and accounted for most of the volume of the gut content (Figure 5 . 6B) . The TCM section is the place for harboring the symbiotic bacteria The SCM section bore a number of crypts with thick epithelia, and most of the gut content was present not in the crypts but in the main tract of the midgut section (Figure 5 6C) The glandular epithelial cells were rich in microvilli and secretion granules containing electron-dense materials The materials were secreted into the gut cavity, forming a secretion matrix embedding the symbiont cells (Figure 5 . 6D) . Hence, the SCM section is the organ specialized for production of the matrix. The BEM section consisted of deep and narrow crypts, whose cavity was filled with filament-like materials (Figure 5 . 6E) . The fine structure of the materials (Figure 5 . 6F) was identical to that of the envelope of the symbiont capsule (Figure 5 4B) Hence, the BEM section is the organ specialized for production of the cuticular envelope materials

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