日本におけるハマダイコンの葉毛密度の地理的変異

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日本におけるハマダイコンの葉毛密度の地理的変異

著者 Han Qingxiang, Sakaguchi Shota, Setoguchi Hiroaki

著者別表示韓慶香, 阪口翔太, 瀬戸口浩彰

journal or

publication title

The journal of phytogeography and taxonomy

volume 64

number 1

page range 17‑23

year 2016‑09

URL http://doi.org/10.24517/00053292

Creative Commons : 表示 ‑ 非営利 ‑ 改変禁止 http://creativecommons.org/licenses/by‑nc‑nd/3.0/deed.ja

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Abstract

Understanding the geographic variation of leaf trichome in wild radish accessions would help to elucidate how morphological variation can be molded by natural selection. Here, we investigated the geographic variation of leaf trichomes in wild radish populations across Japan covering Ryukyu Islands and mainland. And we also sought the possible selection pressures involved in the evolution and maintenance of leaf trichome polymorphism. Results showed leaf trichome density in wild radish was highly variable across the Japanese archipel- ago, with plants from southern insular populations tending to exhibit glabrous leaves, while those of northern mainland Japan were completely covered by dense trichomes. This trichome variation in Japanese populations of wild radish was probably attributing to external stress involving in protecting against frost damage and resistance to herbivores. This study will be helpful to understand the morphological variation under divergent selections in wild radish and contribute to the areas of breeding and evolutionary studies in radish.

Key words : Divergent habitats, Geographic variation, Selection, Trichome, Wild radish.

1Graduate School of Human and Environmental Studies, Kyoto University, Yoshida Nihonmatsu-cho, Sakyo-ku, Kyoto, Japan （^＊Corresponding author: Tel: +81-075-753-6860 Fax: +81-075-753-6694 E-mail: han.qingxiang.88r@

st.kyoto-u.ac.jp）

Introduction

Trichomes are hair-like appendages that appear on the surfaces of leaves, stems, petioles, and sepals, as well as on the leaf margin of most land plants （Johnson 1975; Werker 2000）. They contribute to protection against herbivorous insects （Agren and Schemske 1994; Levin 1973; Traw and Dawson 2002; Valverde et al.

2001）. The glabrous morph of the perennial herb Arabidopsis is more susceptible to dam- age by insect herbivores than is the trichome- producing morph （Løe et al. 2007）, and ex- perimental removal of leaf trichomes resulted in increased feeding and growth of herbivorous insects in bioassay studies of several species

（Baur et al. 1991; Fordyce and Agrawal 2001）. In addition, some plant species respond to herbivores by producing new leaves with an increased density or number of trichomes, and insects feeding on the induced plants often con- sume less foliage and grow less compared with insects feeding on the glabrous plants （Agrawal

2000; Dalin and Björkman 2003）. Trichomes may also function to protect plants from abiotic stresses such as drought by reflecting light and reducing transpiration （Ehleringer 1984; Pérez- Estrada et al. 2000）, and protecting cells from damage caused by solar UV radiation （Skaltsa et al. 1994）, temperature stress （Agrawal et al.

2004）, and heavy metal toxins （Gutiérrez-Alcalá et al. 2000）.

Raphanus sativus L. var. raphanistroides Makino （Family: Brassicaceae） is a winter annual plant known as wild radish. It is a self-in- compatible and insect-pollinated wild plant that grows primarily on dunes or sandy beaches near the sea in East Asia. A number of studies have measured variation in life-history （e.g.

trichomes, leaf size, and emergence time） and flower traits （e.g. corolla width, pollen production per flower, and pollen size） in wild radish

（Agrawal et al. 2002; Conner and Via 1993;

Williams and Conner 2001; Young et al. 1994）. Among these traits, trichomes show morpho-

Qingxiang Han

^1,＊

, Shota Sakaguchi

¹

, Hiroaki Setoguchi

¹

: Geo-

graphic variation in leaf trichome of wild radish （ Raphanus sativus L. var. raphanistroides Makino ） in Japan

Journal of Phytogeography and Taxonomy 64：17-23, 2016

©The Society for the Study of Phytogeography and Taxonomy 2016

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Locations Codes Samples

No. Glabrous/

Hairy Percentage

(glabrous) Trichome density

(Mean±SD) T1

（°C） T2

（°C） Latitude

（°N） Ryukyu Islands

Iriomote Isl. 1 16 9/7 56.25% 134.29±32.33 22.7 14.2 24.27

Miyako Isl. 2 33 23/10 69.69% - 23.5 15.3 24.73

Okinawa Isl. 3 24 23/1 95.83% - 22.3 12.9 26.44

Yorontou Isl. 4 18 17/1 94.44% - 22.0 12.8 27.02

Tokunoshima Isl. 5 17 14/3 82.35% - 21.0 12.0 27.77

Amamioshima Isl. 6 17 14/3 82.35% - 21.5 11.4 28.45

Mainland Japan

Kochi Pref. 7 19 0/19 0% 144.83±46.52 15.4 2.3 33.23

Mie Pref. 8 16 0/16 0% 58.38±30.39 15.9 1.3 34.60

Shiga Pref. 9 16 0/16 0% 164.89±60.06 14.4 -1.4 35.42

Aomori Pref. 10 17 0/17 0% 114.00±23.19 9.7 -5.4 40.58

Hokkaido Isl. 11 19 0/19 0% 123.74±32.05 3.5 -11.5 41.95

T1: Annual mean temperature; T2: Minimum temperature of coldest month. Population codes correspond to those of Fig. 1.

Table 1. Types and trichome density on leaf surface of wild radish and the environmental information of their population localities.

Fig. 1. Sampling locations of wild radish populations.

植物地理・分類研究 第 64 巻第 1 号 2016 年 9 月

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logical variations that mediate adaptive expres- sion in different environments. Understanding the geographic variation of leaf trichome in wild radish accessions would help to elucidate how morphological variations can be molded by selection. Here, we investigated trichome polymorphisms in wild radish populations and dis- cussed the possible selection pressures involved in the shape and maintenance of polymorphism in natural populations. This study will be helpful to understand the morphological variation under divergent selections in wild radish and contribute to the areas of breeding and evolutionary studies in radish.

Materials and methods

We collected seeds of wild radish in the field in the Ryukyu Islands （Iriomote and Okinawa Islands: populations 1 and 3, respectively, in Fig. 1） and mainland Japan （Kochi, Mie, Shi- ga, Aomori prefectures, and Hokkaido Islands:

populations 7, 8, 9, 10, and 11, respectively, in

Fig. 1）, and cultivated them in a greenhouse to investigate their leaf types. In addition, four populations （Miyako, Yorontou, Tokunoshima, and Amamioshima Islands: populations 2, 4, 5 and 6, respectively, in Fig. 1） were investigated in the fi eld. The annual mean temperature and the minimum temperature of the coldest month at each population locality are provided in Table 1. We recorded the presence or absence of trichomes for each plant. Here, we classi- fi ed all plants with trichomes on the leaf blade as “trichome-producing” and plants without trichomes on the leaf blade as “glabrous.” Gen- erally, trichome variation could be considered a discrete （glabrous vs. trichome-producing plants） or a quantitative （trichome density or number on trichome-producing individu- als） trait. For trichome-producing accessions, high density of trichome occurs on the adaxial side of leaf, whereas, only minimal trichomes present on the vein on the abaxial side （Fig.

2）. Here, trichomes were counted on the ad- Fig. 2. Trichome variations within wild radish. （a） adaxial side and （c） abaxial side of glabrous leaf of wild

radish from the Ryukyu Islands; （b） adaxial side and （d） abaxial side of trichome-producing leaf of wild radish from mainland Japan.

September 2016 J. Phytogeogr. Taxon. Vol. 64. No. 1

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axial side of each fully expanded leaf （60 days growth） at a defined measuring area of 1 cm², avoiding the main leaf vein. Microscopic images of trichomes on each leaf were taken using a digital image input device （3CCD 2Mega Pixel Digital Camera FX630, Olympus Optical Co., Ltd., Tokyo, Japan）, and the total number of trichomes per unit area was counted. The rela- tionships between latitude and trichome density and between latitude and leaf type （defined as trichome-producing （1） or glabrous （0）） were evaluated by fitting linear and logistic regression models, respectively, using the R package

（Zeileis et al. 2007）.

Results

Leaf trichome density in wild radish was highly variable across the Japanese Islands.

Plants from the Ryukyu Islands exhibited mostly glabrous leaves whereas those of mainland Japan were totally covered by dense trichomes （Fig. 2）. The percentage of plants from the Ryukyu Islands producing glabrous leaves was greater than 56.25% （Iriomote Isl.）, while accessions from mainland Japan were entirely

trichome-producing （Table 1）. Trichome density in one population from the Ryukyu Islands

（Iriomote） and five from mainland Japan were investigated. Among trichome-producing populations, the average trichome density from Iriomote was 134.29, while the mean value in each population from mainland Japan ranged from 58.38 （Mie pref.） to 164.89 （Shiga pref.）

（Table 1）. There was no significant relationship between latitude and trichome density （p > 0.05）, but a significant relationship was observed between latitude and leaf types （glabrous vs. tri- chome-producing） for all populations （ p < 0.001;

Fig. 3）, suggesting distinct leaf types of accessions from the Ryukyu Islands and mainland Japan.

Discussion

Wild radish trichome polymorphisms are both widespread and variable among different accessions in Japan. Trichome-producing plants occurred mostly in mainland Japan, whereas glabrous ones were found only in the Ryukyu Islands. In addition, the observed divergence in trichome characteristics was consistent with Fig. 3. The relationship between latitude and leaf type analyzed by fitting a linear model. Circles present leaf

type: 1 for trichome-producing leaf; 0 for glabrous leaf （see the left vertical axis）. Bars mean the numbers of samples investigated for each leaf type （see the right vertical axis）.

植物地理・分類研究 第 64 巻第 1 号 2016 年 9 月

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the previously reported genetic variation of wild radish populations observed in the Ryukyu Islands and mainland Japan （Han et al. 2015）. Furthermore, the Ryukyu Islands and mainland populations appear to be genetically isolated over 14 nucleotide loci （Han et al. 2016）. Therefore, we hypothesize that the difference in trichomes between the Ryukyu Islands and mainland wild radish populations is probably genetic.

At the population level, leaf type varies ac- cording to the microenvironment of the plant.

The Ryukyu Islands are near mainland Japan but are isolated geographically by barriers such as the Tokara and Ohsumi Straits （Feng et al.

2000; Millien-Parra and Jaeger 1999）. These regions experience different climates, which in turn may produce different selection pressures on local populations. A previous study reported that wild radish leaf trichomes could provide a protective function against low temperatures

（Agrawal et al. 2004）. Our study supports this hypothesis, as the trichome-producing accessions were found mostly in mainland Japan, which has a much cooler climate than that of the Ryukyu Islands. However, trichomes appear to also provide mechanical defense. Tri- chomes have been shown to reduce herbivory in several plant species, including Raphanus raphanistrum （Agrawal 1999）, Arabidopsis ly- rata （Løe et al. 2007）, Brassica rapa （Agren and Schemske 1993）, Alnus incana （Baur et al.

1991）, and Brassica nigra （Traw and Dawson 2002）. Agrawal （1999） investigated the induced responses to herbivory in annual wild radish plants and showed that leaf herbivory resulted in decreased herbivore growth, which correlated with an increase in trichomes. Based on this information, we could assume that trichome production in wild radish probably provides structural defense against herbivores, although our study could not provide explicit evidence that could support this hypothesis.

In conclusion, our study was reported geographic variation in leaf trichome in wild radish and indicated that divergent habitat factors and herbivory may be important determinants of trichome variations. Future studies should aim to unravel the environmental factors under-

lying this intra-specific morphological variation and determine how these variations are medi- ated genetically. Further study is necessary to investigate the multiple genes associated with trichome variation. Improving our knowledge on these issues will provide a comprehensive understanding of the genetic and environmental sources for morphological variation in wild radish.

Acknowledgements

This work was supported by a Grant-in- Aid for Science Research （no. 24247013 and 25128708） to H.S.

日本におけるハマダイコンの葉毛密度の地理的変異

韓慶香・阪口翔太・瀬戸口浩彰

京都大学 大学院人間・環境学研究科（606-8501;

京都市左京区吉田二本松町）

ハマダイコンは日本においては西表島から北海道 までの幅広い緯度の海岸に生育する。著者らの従来 の観察では，琉球列島では葉や茎が無毛である傾向 があり，反対に本州北部では粗長毛が密に生える傾 向が見られた。そこで本研究では曖昧な印象であっ た毛の密度について調べるために，日本列島内の緯 度に沿った11集団に由来する個体の，葉の向軸側 の毛の密度を実体顕微鏡で計測した。その結果，琉 球列島だけにおいて無毛の個体が存在するととも に，少数ながら粗長毛が密に生える個体も共存し た。その一方で，北緯33度以上の本州の集団では すべての個体で葉に粗長毛が密に生えていた。近年 のアブラナ科植物での研究では，毛が草食性昆虫な どの食害を防ぐなどの役割があると報告されてお り，今後には，なぜ南西諸島の集団で無毛の個体が 共存できるのかという検討が必要である。

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植物地理・分類研究 第 64 巻第 1 号 2016 年 9 月

日本におけるハマダイコンの葉毛密度の地理的変異