Weishuang TONG，Zhanbo GUAN，Huashan GAO

College of Chemical and Environmental Engineering，Pingdingshan University，Pingdingshan 467000，China

Abstract China is not only one of the origin centers of Pyrus L.，but also the earliest birthplace of Pyrus L. in the world. This paper reviews the evolution of Pyrus L. from the aspects of leaf edge morphology，inflorescence and fruit type，and summarizes the research progress of classification and species distribution of Pyrus L.，which is of great significance for the protection，evaluation and utilization of germplasm resources.

Key words Pyrus L.，Origin，Evolution，Classification，Species distribution，Germplasm resources conservation

1 Introduction

PyrusL.，as deciduous trees or shrubs，falls within dicotyledonous class，Rosales order，Rosaceae family. The fruit is sour，sweet and delicious，and its nutritional value is high. It has been one of the favorite fruits since ancient times. Because of the white and beautiful pear flowers，PyrusL. is often used in garden cultivation. The fruits ofPyrusL. can also be used as medicine，which can clear the lungs and resolve phlegm，invigorate the body and quench thirst，help digestion，nourish yin and moisten the lungs，relieve sore poison，alcohol poison and so on. There are about 25 species ofPyrusL. in the world，which are distributed in Asia，Europe and North Africa. There are 14 species in China，8 of which are endemic to China. China is not only one of the origin centers ofPyrusL.，but also the earliest birthplace ofPyrusL. in the world. The study on the origin，evolution，classification and species distribution ofPyrusL. is of great significance to the protection，evaluation and utilization of germplasm resources.

2 Study on the origin

China is not only one of the origin centers ofPyrusL.，but also the earliest birthplace ofPyrusL. in the world. It is reported thatPyrusL. originated in the mountains of western China in the Cenozoic era. Due to the geographical isolation ofPyrusL. community and its adaptability to cold and dry conditions，it developed to both sides along the mountain range，which promoted its evolution. The westward evolution ofPyrusL. is mainly in the European direction，forming western pears，while the other side is mainly in eastern Asia，especially in China，forming oriental pears. At the same time，it has also formed the three origin centers of cultivated pears in the world，namely，the center of China，the center of Central Asia and the center of the near East. Yu Dejun，a plant taxonomist at the Institute of Botany of the Chinese Academy of Sciences，believes that western pears originated in central and southeastern Europe，as well as in Asia minor and Central Asia，and were cultivated as early as 2 000 years ago in Roman times.

In addition，American scholars combined 29 chemical substances with 22 botanical characteristics to build a classification chart composed of computer quantitative analysis methods，and listedP.calleryanaas the most primitive species in the evolutionary classification diagram. According to the characteristics of pollen and the results of karyotype research onP.calleryana，some scholars also listedP.calleryanaas the most primitive species.

3 Evolution

3.1 Evolution of leaf edge morphologyFor thePyrusL. in China，the primitive type is often serrated round or obtuse or entire，while the evolutionary type is prickly at the serrated apex of the leaf margin. WesternPyrusL. is a special type. The leaf edges of most species ofPyrusL. in China are serrated，but the types of serration are different. The common leaf edge serration types are shown in Table 1. The upper leaf edge ofP.xerophilahas a few fine and sharp edges，the lower leaf edge ofP.sinkiangensisYuis not serrated or has shallow obtuse edges，and the upper leaf edge is mostly fine and sharp. The results show thatP.xerophilaandP.sinkiangensisYu evolved from east to north in the transitional stage of evolution.

3.2 Evolution of inflorescencesThe evolution trend of inflorescences ofPyrusL. in China is as follows: from raceme or racemose-corymbose to corymbose and then to subumbellate，as shown in Table 2. In terms of the number of flowers per inflorescence，it decreases gradually in the above order. The diameter of each flower tends to increase gradually，for example，the diameter ofP.betulacfoliaandP.calleryanaflower is generally less than 3 cm，while that ofP.ussuriensisandP.bretachneideriflower is more than 3 cm，a maximum of 4 cm. The east-west succession and north-south succession ofPyrusL. appeared.

Table 1 Leaf margin serration types of Pyrus L. in China

Table 2 Inflorescence types of Pyrus L. in China

3.3 Evolution of fruit typeIn the types ofPyrusL. in China，those with a fruit diameter of less than 2 cm are called small fruit type，and those with a fruit diameter of more than 2 cm are called large fruit type. The research data show that the small fruit type is more primitive than the large fruit type，the ovaries or fruits with 2-3 locules are more primitive than those with 4-5 locules，and those with brown pericarp are more primitive than those with yellow-green pericarp. The fruit type characteristics ofPyrusL. in China are shown in Table 3.

The pericarp of large fruit type with 2-3 locules is mostly yellow or green，brown，and most of them are cultivars with 5 locules. It is obvious that there is a trend of eastward and northward succession from the Pearl River and the Yangtze River basin and the Himalayas to North China and the Yellow River Basin.

Table 3 Fruit types of Pyrus L. in China

4 Classification study

According to different classification basis，different countries and scholars have made different classifications ofPyrusL.，as shown in Table 4.

Table 4 Classification of Pyrus L.

5 Species distribution

There are about 25 species ofPyrusL. in the world，which are distributed in Asia，Europe and North Africa. There are 14 species in China，8 of which are endemic to China. They are widely cultivated in the north and south of China.

5.1 Distribution in the worldAccording to the research of American scholar Westwood，PyrusL. in the world can be divided into four major populations，as detailed in Table 5.

Table 5 Distribution of Pyrus L. populations in the world

5.2 Distribution in ChinaThe species distribution ofPyrusL. in China is shown in Table 6.

Table 6 Distribution of Pyrus L. populations in China

6 Conclusions

PyrusL. is widely distributed and adaptable in China，and China is also one of its origin centers. The study on the origin，evolution，classification and species distribution ofPyrusL. is of great significance for the protection，evaluation and utilization ofPyrusL. germplasm resources.