藉由比較解剖探討桑科榕屬榕果果壁的防禦功能性特徵

Abstract

非授粉蜂與授粉蜂競爭榕果的資源或以授粉蜂的幼蟲為食，減低授粉蜂與榕樹的適度，因此非授粉蜂對榕樹與授粉蜂的互利共生關係有負面的影響。大部分非授粉蜂在榕果表面探測宿主，接著以產卵管刺穿榕果果壁在榕果內產卵。因此，榕果果壁是抵抗非授粉蜂行外部產卵的第一道防線。然而，前人研究主要探討非授粉蜂對授粉蜂族群的影響，榕果的防禦機制則鮮少被研究。本研究的目的為檢驗台灣各種榕樹榕果果壁的結構，進而探討防禦機制的效用。實驗材料為台灣及蘭嶼採集到的六個亞屬22種榕樹（24個分類單位），藉由組織染色處理埋蠟切片及徒手切片，以分析榕果果壁的組織結構。研究結果顯示有17種的榕果果壁有進一步分化，包含通氣組織（aerenchyma）的形成及厚壁化（sclerification）。依據榕果的果壁結構可將22種榕果分成四型，分別為第一型（無進一步分化）、第二型（具通氣組織）、第三型（厚壁化）以及第四型（具單寧細胞）。此外，雌雄異株的榕樹中，部分種類雌雄果具有相異的果壁結構，雌果為第一型、雄果為第三型；反之，雌雄果果壁結構相同的種類皆為第一型或第二型。其次，以雌雄同株的大葉雀榕、雀榕及正榕進行榕果各發育期的果壁形態比較，結果發現厚壁化的發生時間點皆稍早於類寄生蜂與依附性造癭蜂造訪榕果的時期，且正榕上的單寧細胞自發育早期（A期末）形成後便持續到榕果成熟前（D期）。總結，研究結果顯示榕果可以利用機械性（厚果壁、通氣組織和厚壁化）及化學性防禦（單寧）抵抗非半翅目的植食性昆蟲。趨勢上呈現，雌雄同株具有厚壁化的薄果壁榕果有較多的非授粉蜂種類，例如大葉雀榕、雀榕及正榕；雌雄異株具有通氣組織的厚果壁榕果則有較少的非授粉蜂種類，例如薜荔。本研究可提供一個新的層面以回答：關鍵性特徵是如何在榕果、授粉蜂與非授粉蜂之間共演化。

Nonpollinating fig wasps (NPFWs) compete pollinating fig wasps over the resource provided by figs or feed on pollinating fig wasp larvae. Therefore, they negatively affect the fig-fig wasp mutualism by reducing the fitness of pollinating fig wasps and fig trees. Most of the NPFWs probe on the fig surface and oviposit through the fig wall from the outside of the figs. Thus, the fig wall serves as the first defense structure against the external oviposition by NPFWs. Previous studies mainly explored the effects of NPFWs on pollinating fig wasps. However, few studies investigated the defensive mechanism in figs. Therefore, the first part of this study aims to examine fig wall structure among species in Taiwan and then explores the efficacy of defensive functional traits. To compare the tissue structure of the fig wall on 22 Ficus species (24 taxa) from six subgenera in Taiwan, paraffin and free-hand sections were made and histochemically stained. The results showed that 17 fig walls of the 24 sampled taxa underwent further differentiation, including aerenchyma formation and sclerification. Based on the fig wall structure, four types of fig wall were described as follows: type I (no further differentiation), type II (aerenchyma formation), type III (sclerification), and type IV (tannin deposition). The fig walls of dioecious species either shared similar anatomical traits (type I or II) between female and male figs, or presented different types (e.g. type I in female fig walls and type III in male fig walls). For the second part, fig walls of three monoecious species, Ficus caulocarpa, F. subpisocarpa, and F. microcarpa, with five developmental phases were compared. The results showed that sclerification happened before the visitation by parasitoids and inquilines. In F. microcarpa, tanninferous cells formed at early developmental stage (late A phase) and degraded before ripening (D phase). In conclusion, the mechanical (thick fig wall, aerenchyma formation, and sclerification) defenses deter NPFWs from ovipositing, whereas the chemical defenses (tannin deposition) may target insect non-hymenopteran herbivores. Monoecious figs with sclerified thin fig wall were associated with more NPFW species (e.g. F. caulocarpa, F. microcarpa, and F. subpisocarpa). Dioecious figs with thick type II fig wall were associated with less NPFW species (e.g. F. pumila var. pumila). This study can provide a new aspect to answer how key traits have coevolved among figs, pollinators and NPFWs.