風媒のスペシャリスト、ヤマトグサ属 Specialist of wind-pollination Theligonum

ヤマトグサ Theligonum japonicum: (A-C) ヤマトグサの開花個体(A)、花の拡大(B)、未開花個体(C) 。
図1 ヤマトグサ Theligonum japonicum: (A-C) ヤマトグサの開花個体(A)、花の拡大(B)、未開花個体(C) 。

ヤマトグサ属Theligonumは、4種が知られている。日本と中国東部にヤマトグサ Theligonum japonicum Okubo & Makino (Chen and Funston 2011)、台湾にT. formosanum (Ohwi) Ohwi & T.S. Liu (Chen and Funston 2011)、中国中西部(四川省、雲南省など)にT. macranthum Franchet (Chen and Funston 2011)、地中海沿岸にT. cynocrambe L. (https://www.gbif.org/species/2919295)、が分布する。

Four species of the genus Theligonum are known: Theligonum japonicum Okubo & Makino (Chen and Funston 2011) in Japan and eastern China, T. formosanum (Ohwi) Ohwi & T.S. Liu (Chen and Funston 2011) in Taiwan, T. macranthum Franchet (Chen and Funston 2011) in central and western China (Sichuan Prov, and Yunnan Prov.), T. macranthum Franchet (Chen and Funston 2011) in central and western China (Sichuan, Yunnan, etc.), T. cynocrambe L. (https://www.gbif.org/species/2919295) on the Mediterranean coast.

(A-C) T. japonicum: Flowering individuals (A), enlarged flowers (B), and non-flowering individuals (C).

図2 アカネ亜科の分子系統樹


The genus Theligonum belongs to the family Rubiaceae, subfamily Rubioideae, and section Theligoneae. The section Theligoneae includes only the genus Theligonum.

ヤマトグサの栄養シュートと生殖シュート Vegetative and reproductive shoots of Theligonum japonicum
図3 ヤマトグサの栄養シュートと生殖シュート Vegetative and reproductive shoots of Theligonum japonicum


T. japonicum is monoecious, with the same individual forming male and female flowers (A, B). In the vegetative shoot, the leaves are opposite (C, D), as in other Rubiaceae species. On the other hand, in the flowering reproductive shoot, two male flowers are formed in the opposite position to the leaf (E, F). The male flowers are hypothesized to be formed in the axils of the degenerated and lost leaves.

ヤマトグサの雄花  Male flowers of Theligonum japanicum
図4 ヤマトグサの雄花 Male flowers of Theligonum japanicum

成熟した雄花の蕾の先端をそっと触ると、ほんの数秒で、1輪のみある花被が背側(外側)に反り返り(GからJ:5秒ほどで開いた)、20本程度の雄ずいが垂れ下がる(J)。雄ずい数は花によって変異があり(Rutishauser et al. 1998: 内貴 2017)、花被が4枚なのに、雄ずい数が4の倍数でない場合もある。アカネ科の他の種では、雄ずい数は種内で一定の場合がほとんどであるが、ヤマトグサでどうして雄ずい数が変異するのかはよくわかっていない(Rutishauser et al. 1998)。

When the tip of a mature male flower bud is gently touched, in just a few seconds the single whorl perianth turns abaxially (outward) (G to J: opened in about 5 seconds), and about 20 male stamens hang down (J). The number of stamens varies from flower to flower (Rutishauser et al. 1998: 内貴 2017), and the number of stamens may not be a multiple of 4, even though the flower has 4 perianths. In most other species of the family Rubiaceae, the number of stamens is constant within a species, but it is not well understood why the number of stamens varies in the genus Theligonum (Rutishauser et al. 1998).

ヤマトグサの葯糸 Filaments of Theligonum japonicum
図5 ヤマトグサの葯糸 Filaments of Theligonum japonicum


The filaments of stamens (K, L; L is an enlargement of the black dotted area of K) is a wiggly array of odd cells (M) with ridges and longitudinal stripes, about 50 µm in diameter and thin. They are soft and mobile, but strong and hard to break. Thus, when the wind blows, the anthers wobble and effectively disperse pollen.

ヤマトグサの雌しべ Female flower of Theligonum japonicum
ヤマトグサの雌しべ Female flower of Theligonum japonicum
図6 ヤマトグサの雌しべ Female flower of Theligonum japonicum

雌花は生殖シュートの葉の葉腋に形成される(A、B、N-Q)。(Q、R)子房下位だが、花被と雌ずいは、他の子房下位の花のように子房の先端ではなく、子房の側面下方から伸長する。花被は1輪だけ形成され、花弁と萼片のどちらが消失したかはわかっていない。他のアカネ科の種では萼片は果時まで宿在し、花弁は早落する場合が多く (Robbercht 1988)、Theligonumの花被は、早落性であることから、花弁ではないかという仮説が提唱されている(Rutishauser 1998)。一方、花弁が退化して萼片のみが残っているという見解(内貴 2017)もある。萼片で働く遺伝子、花弁で働く遺伝子のどちらが発現しているかなど、さらなる研究が必要である。

花柱(QとRの赤色星印)は花被(Qの黒色星印、R)に覆われる部分のみで、乳頭状細胞を持つ柱頭が長く伸長する。子房は発生初期は2室だが、1つの胚珠のみが発達し(S、T)、最終的に1室となる(Rutishauser et al. 1998)。

ヤマトグサ属は、他の多くのアカネ科の種と比較して、(1)葉序が常に対生ではなく、栄養シュートは対生だが、花序は互生へと転換すること、(2)虫媒ではなく風媒anemophyllyであること、(3)雄花、雌花ともに、花被が2輪ではなく1輪であること、(4)雄花が葉腋ではなく、葉腋の反対側から生じること、(5)雄花の雄ずいが1輪の花被と同数ではなく、2から19本もあること、(5)花柱がほとんど無く、円柱状に伸びた柱頭が子房基部から側生すること、(6)子房が2室が癒合して1室となり1胚珠のみ形成されることなどの点が異なっている(Rutishauser et al. 1998)。ただ、どの形質もアカネ科の中で見られる形質であり、アカネ科に含めるのが妥当であると提唱されたが(Wunderlich 1971)、異論もあった(Praglowski 1973, Nowicke and Skvarla 1979)。しかし、詳細な形態比較(Rutishauser et al. 1998)、分子系統解析(Bremer 2009)から、アカネ科のアカネ連Rubieae、ケロギア属Kelloggia、プトリア連Putorieaeと単系統群であることがわかった。ハシカグサに葉の外観が似るが、花はもとより、托葉形態、葉の毛の生え方が異なっており、分子系統解析からも姉妹関係ではない(Bremer 2009, Neupane et al. 2015)。

Female flowers are formed in the leaf axils of the reproductive shoot (A, B, N-Q). (Q, R) The ovary is inferior, but the perianths and pistil are formed not at the tip of ovary but the lateral side.

The perianth is transparent, membranous, and trace-like. The petals are thinner than the sepals in other Rubiaceous species, leading to the hypothesis that the sepals are degenerated (Rutishauser et al. 1998) or that the petals are degenerated and only the sepals remain (内貴 2017), but which is valid is not known. Further research is needed to determine which genes are expressed, those working in the sepals or those working in the petals.

The style is the only part covered by the petiole, with a long elongated stigma with papillose cells. The ovary is biloculate in early development, but only one ovule develops (S, T) and eventually becomes unilocular (Rutishauser et al. 1998). Compared to many other species in the family Rubiaceae, the genus Theligonum has the following special characteristics: (1) the leaves are not always opposite, the vegetative shoot is opposite, but the inflorescence converts to alternate, (2) it is wind pollinated (anemophily) rather than insect pollinated, (3) both male and female flowers have a single perianth whorl, (4) the male flowers are produced on the opposite side of the leaf, and no subtending leaf, (5) the number of stamens are not equal to that of perianths (four perianths), but 2 to 19, and (6) The ovary is bilocular but changes to actually unilocular with a single ovule. (Rutishauser et al. 1998). However, all of the traits as evolutionary trends are found in other Rubiaceae (Rutishauser et al. 1998), and it was proposed that they should be included in the family (Wunderlich 1971), but there were other hypotheses (Praglowski 1973, Nowicke and Skvarla 1979). However, detailed morphological comparisons (Rutishauser et al. 1998) and molecular phylogenetic analysis (Bremer 2009) have shown that Theligonum forms a clade with Rubieae, Kelloggia, and Putorieae. Although the leaves are similar in appearance to those of Neanotis hirsuta in the same family, morphology of the flowers, leaf and leaf hair are different, and molecular phylogenetic analysis indicates that they are not sister taxa (Bremer 2009, Neupane et al. 2015).

アカネ科で風媒花のCoprosma repens Wind pollinated flowers of Coprosma repens in the Rubiaceae
図7 アカネ科で風媒花のCoprosma repens Wind pollinated flowers of Coprosma repens in the Rubiaceae

アカネ科では、ヤマトグサ連の種以外に、Anthospermeae連で風媒花が知られている。Anthospermeaeに属する種は、丁字着葯で、葯糸が長く、垂れ下がった雄ずいを形成し(Rutishauser et al. 1998)、風媒花であると考えられている(Puff 1982, Thureborn et al. 2019)。アカネ科でヤマトグサ属以外は虫媒であると考えられていることから(Puff 1982, Thureborn et al. 2019)、Anthospermeaeの共通祖先で虫媒から風媒への進化がおこったと推定できる。

Anthospermeaeの種と比較して、ヤマトグサ属の種は、風媒に寄与する「多数の」形質が進化している点で特異である。多数の雄ずい、細長くて丈夫な花糸、細長く開花後すぐに裂開する葯、反り返って雄ずいを露出させる花弁、花柱が短く柱頭が伸長した雌ずい、退縮した雌しべの花被、雄ずいが形成される節には雄ずい側に葉が形成されず、雄ずいの花粉が散布されやすくなっている生殖シュートなどである。複合的な形質の進化には、超遺伝子形成が関わっていた可能性が考えられている(総説として、藤原 2023)。ヤマトグサ属の風媒に関わる複数の形質がどのような遺伝子によって制御されているか、それらの遺伝子がゲノム上でどのように配列しているかを研究することで、複数の適応形質の進化について新しい視点からの研究が進展する可能性がある。また、それぞれの形質はアカネ科の他の節でも見られる形質であり、アカネ科の共通祖先が持っていた遺伝子系が外適応として進化した可能性もある。

In addition to the species of Theligonum, wind-pollinated flowers are known in the tribe Anthospermeae of the family Rubiaceae; species belonging to the Anthospermeae have dorsifix anthers, long filaments, and pendulous stamens (Rutishauser et al. 1998) and are considered to be wind-pollinated (Rutishauser et al. Puff 1982, Thureborn et al. 2019). Since all members of the Rubiaceae are considered to be insect-pollinated (Puff 1982, Thureborn et al. 2019), it can be inferred that wind-pollinated flowers evolved at the common ancestor of the Anthospermeae from ancestral insect-pollinated flowers.

Compared to Anthospermeae species, Theligonum species are unique in that they have evolved multiple traits that contribute to wind pollination. These include numerous stamens, long and strong filaments, long and slender anthers that split open soon after flowering, tepals that are recurved to expose the stamens, pistils with the short style and the elongated stigma, reducted pistil perianths, and reproductive shoots in which the nodes where the male flowers are formed have no leaves on the male flower side, facilitating the dispersal of pollen. It is hypothesized that supergene formation may have been involved in the evolution of complex traits (for a review, 藤原 2023). Studying what genes control the multiple traits involved in wind pollination in Theligonum, and how these genes are arranged in the genome, may advance new perspectives on the evolution of multiple adaptive traits. Each of these traits is also a trait found in other sections of the Rubiaceae, and it is possible that the genetic system possessed by the common ancestor of the Rubiaceae evolved as exaptation.


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https://www.gbif.org/species/2919295: GBIF Secretariat: GBIF Backbone Taxonomy. https://doi.org/10.15468/39omei Accessed via https://www.gbif.org/species/2919295 on 26th August 2023

カテゴリー: 0.3.0.Leaf 葉,リンドウ目, Modification for pollination, Perianths 花被, Phyllotaxy 葉序, pollination, Reduction of floral organs and bracts, Rubiaceae アカネ科, Stamen morphology 雄蕊形態 パーマリンク