Maruyama Tadashi, Hirose Euichi, Ishikura Masaharu
Marine Biotechnology Institute, Kamaishi Laboratories, Heita 3-75-1, Kamaishi, Iwate 026-0001, Japan.
Biol Bull. 2003 Apr;204(2):109-13. doi: 10.2307/1543546.
Coral reef invertebrates that host phototrophic symbionts are thought to protect themselves and their symbionts with mycosporine-like amino acids (MAAs)-UV-absorbing substances that act as sunscreens (Dunlap, W. C., and J. M. Shick, 1998. J. Phycol. 34: 418-430). However, the histological distribution of MAAs in the host tissues has not yet been visualized. We have localized the UV-absorbing substances in the tissues of two colonial didemnid ascidians-Lissoclinum patella and Diplosoma sp.-that contain the symbiotic photo-oxygenic prokaryote Prochloron sp. Cross-sections of unfixed tissue from these ascidians were examined by UV-light microscopy at 320 or 330 nm, wavelengths at which UV light is absorbed by MAAs. Within the tunic, the gelatinous integument of the colony, UV light was exclusively absorbed by a particular type of cell, the tunic bladder cell. Tunic bladder cells with strong UV absorption were denser in the upper tunic, which lies over a colony's zooids, than in the basal tunic underlying the zooid. In the upper tunic, those cells with strong UV absorption were most dense near the surface. The tunic bladder cell is highly vacuolated, and the vacuole contains strong acid, which destabilizes MAAs. Furthermore, the UV-absorbing portion of tunic bladder cells seemed to be cup-shaped, indicating that the MAAs are not localized in the vacuole, but in the cytoplasm. These results strongly suggest that didemnid ascidians accumulate MAAs in tunic bladder cells as a protection against UV radiation.
宿主光合营养共生体的珊瑚礁无脊椎动物被认为是用类菌孢素氨基酸（MAAs）——一种作为防晒剂的紫外线吸收物质来保护自己及其共生体（邓拉普，W.C.，和J.M.希克，1998年。《藻类学杂志》34卷：418 - 430页）。然而，MAAs在宿主组织中的组织学分布尚未可视化。我们已经在两种群体海鞘——柄海鞘和双体海鞘属——的组织中定位了紫外线吸收物质，这两种海鞘含有共生的光合放氧原核生物原绿藻属。通过在320或330纳米的紫外光显微镜检查这些海鞘未固定组织的横截面，MAAs在这些波长下会吸收紫外光。在群体的被囊，即凝胶状外皮中，紫外光仅被一种特殊类型的细胞——被囊膀胱细胞吸收。具有强紫外线吸收能力的被囊膀胱细胞在上层被囊中密度更大，上层被囊位于群体的游动孢子之上，而在游动孢子下方的基部被囊中则密度较小。在上层被囊中，那些具有强紫外线吸收能力的细胞在表面附近最为密集。被囊膀胱细胞高度液泡化，液泡中含有强酸，这会使MAAs不稳定。此外，被囊膀胱细胞的紫外线吸收部分似乎呈杯状，这表明MAAs并非定位在液泡中，而是在细胞质中。这些结果有力地表明，海鞘将MAAs积累在被囊膀胱细胞中以抵御紫外线辐射。
