Chromosome Studies of Chinese Species of Malus Mill.

Abstract

Abstract:

The genus Malus Mill. of the Maloideae of the Rosaceae consists of

about 35 recognized species, which are distributed in temperate region of the

Northen Hemisphere including Asia, Europe and North America. Redher's

classification divided the genus Malus into 6 series, 5 sections, of which 5 series,

3 section, including 27 species, occur in China. The chromosomal counts on 22

Chinese species were reported previously (Chen et al., 1986; Liang, 1987).

However, no comprehensive karyotype analyses have been made for most species

reported. Analyzed in this paper are karyotypes of 36 forms, 22 species, and

synapsis configuration of 6 polyploid forms in Chinese Malus to explore the

relationships among species and provide cytological evidence for delimitation of

species in Malus. Chromosome classification follows Li and Chen (1985). The

materials used are listed in Table 1 and the vouchers are deposited in our depart-

ment. The cytological results are shown as follows (see the data in Table 1,

Plates 1-- 8 and Fig. 1).

1 Sect. Malus

(1) Ser. 1 Baccata The series consists of nine species endemic to China, 7 of

which were cytologically observed: i.e. Malus baccata Borkh. 2n=34; Sooo1-3

of M. baccata Borkh. 2n= 34; M . mandshurica Kom. 2n= 34; M. rockii Rehd.

2n=34; Sooo 3-2 of M. rockii Rehd. 2n=51; M. sikkimensis Koehne.

2n=34; M. hupehensis Rehd. 2n=51; Sooo4-4 of M. hupehensis Rehd.

2n=51; M. halliana Koehne. 2n=34 and M. xiaojinensis Cheng et Jiang

2n=68. The karyotypes consist of sm (8- 9 homologous chromosomes) and

m (6- 7 homologous chromosomes) as well as st (1-2 homologous

chromosomes). The chromosomes range in relative length from 9.84 to 3.74,

with the ratio of the longest to the shortest from 2.28 to 2.03 and the mean ratio

arms from 2.08 to 1.86. All the karyotypes belong to Stebbins's 3B except 2B of

M. Xiaojinensis. Pollen mother cell (PMC) of M. hupehensis contains

17I+17II, and the average synapsis configuration per cell is 2n=3x=51=17.

70I+ 15. 87II+ 0.52III, while PMC meiosis of M. xiaojinensis shows the synapsis

configufations of 2n=4x= 68=4II+ 15IV. The latter has a remarkable and con-

stant deleted chromosome, which is also the smallest in the karyotype.

(2) Ser. 2 Pumilae. The series consists of six species endemic to China (Yu,

1979), all of which were cytologically examined, with the results: Sooo 9-8 of

M. pumila Mill. 2n=34; Sooo 9-12 of M. pumila Mill. 2n=34; Sooo 9-7

of M. sp. 2n=34; M. asiatica Nakai. 2n=68; M. prunifolia Borkh.

2n=34; Soo 12-2 of M. prunifolia Borkh. 2n=34; M. spectabilis Borkh.

2n=51; M. micromalus Makino. 2n=34 and M. sieversii Roem. 2n=34. The

karyotypes consist of sm(8-9 homologous chromosomes) and m (6- 7

homologous chromosomes) as well as st (1-2 homologous chromosomes). The

chromosomes range in relative length from 10.77 to 3.42, with the ratios of the

longest to the shortest from 2.95 to 2.04, and the mean arm ratios from 2.10

to 1.93. Therefore, all the karyotypes belong to 3B. Among more than 20 differ-

ent forms of M. sieversii, there is only one ploidy level (2n=2x=34). PMC

meiosis of M. spectabilis shows the synapsis configurations 2n=3x=51 =5I +

5II+ 12III.

2 Sect. II Sorbomalus

(3) Ser. 3 Sieboldianae. This series has only one species endemic to China,

M. sieboldii Rehd.: One triploid from Zunyi, Guizhou Province has

2n=3x=51=24m+24sm+3st. The chromosomes range in relative length

8.74-- 4.17, with the ratio of the longest to the shortest 2.08, and the mean arm

ratio 1.80. The karyotype, therfore, belongs to 2B. PMC meiosis of this form

shows the synapsis conriguration 2n = 3x = 51 = 6I+ 9II+ 9III.

(4) Ser. 4 Kansuensis consisting of four species endemic to China, 2 of

which were cytologically studied, with the results: M. toringoides Hughes.

2n=34, 51, 68 and M. transitoria Schneid. 2n=34, 51. The karyotypes consist

of m (7—9 homologous chromosomes) and sm (7—9 homologous

chromosomes) as well as a few st chromosomes. The chromosomes range in

relative length from 9.44 to 4.01, with the ratios of the longest to the shortest

from 2.26 to 1.83, and the mean arm ratios from 1.87 to 1.72. In M. toringoides

four forms, 2x, 3x, 4x from Aba, and 3x from Barkang, in Sichuan Province,

are of 2B karyotypes while 2x from Year zhu in Sichuan Province is of 2A. A

mixploid with 3x, 4x in M. toringoides is found from Aba, with one smallest de-

leted chromosome at both levels much like that in M. xiaojinensis. PMC

meiosis of triploid M. toringoides from Barkang showed the synapsis

configurations of 10I+ 10II+ 7III. In M. transitoria 2x, 3x forms from Barkang,

Sichuan Province and Lüda, Liaoning Province belong to 2A, while 2x, 3x

forms from Aba, Sichuan Province both belong to 2B. PMC meiosis of triploid

form from Lüda showed 9I+ 9II+ 8III synapsis configurations.

(5) Ser. 5 Yunnanensis This series consists of four species endemic to China,

3 of which were cytologically studied with the results: M. ombrophila

Hand-Mazz. 2n=34; M. honanensis Rehd. 2n=34, and M. yunnanensis

Schneid. 2n= 34. The karyotypes consist of nine pairs of m, seven pairs of sm

and one pair of st chromosomes. The chromosomes range in relative length from

8.43 to 4.37, with the ratios of the longest to the shortest from 1.88 to 1.86,

and the mean arm ratios from 1.75 to 1.72. Therefore, the karyotypes belong to

2A. Two pairs of heterozygous chromosomes, 1st and 10th, are found carrying a

small satellite.

3 Sect. III Docyniopsis. This section consists of three endemic species to

China, 2 of which were studied with the results: M. formosana Kawak et

Koidz. 2n= 34 and M. melliana Rehd. 2n= 34. Both karyotypes consist of nine

pairs of m, seven pairs of sm and one pair of st chromosomes. The chromosomes

range in relative length from 8.90 to 4.48, with the ratios of the longest to the

shortest 1.85 and 1.83, and the mean arm ratios 1.67 and 1.66. Both karyotyoes

brlong to 2A.

As mentioned above, the karyotyoes of all the species examined are relatively

uniform, i.e. chromosomes small; chromosomes gradually decreasing in size from

the longest to the shortest; chromosomes mostly m and sm; and 1 —2 pairs of st

chromosomes always present. The karyotypes vary in the ratio of the longeat to

the shortest, the ratio of arms and the number of different chromosome types and

satellites. From cytological investigations and previous reports (Chen et al.,

1986; Liang, 1987), the cytology of the genus varies mainly in euploidy variation

and chromosome structure. Intraspecific polyploids and polyploid species make up

58. 1% in 24 species endemic to China reported. The karyotype analyse has

revealed the presence of three major types in Malus: 2A, 2B and 3B. The dele-

tion in some polyploid forms shows another structural variation. It is clear that

the three types are generally consistent with the separation of three sections of

Chinese Malus. According to Stebbins's (1971), the evolutionary trend of

karyotypes is from 2A to 2B and then 3B. The karyotypic data are in accordance

with the evidence from morphology (Langenfeld, 1971; Huckins, 1972) and

biochemistry (Williams, 1982). They both show that the section Docyniopsis and

series Yunnanenses are the most preimitive, while the series Pumilae, Baccatae of

the section Malus are the most advanced or derived. The most typical external

characters in Docyniopsis and the series Yunnanenses e.g. grit-cells in fruit, leaves

folded in bud, and the inflorescence according to Ponomarenko’s (1986) classifica-

tion, might be very primitive. The 2A karyotypes in the genus correlated with

many typically premitive characters. From the data mentioned above, we may

suppose that the series Sieboldianae and Kansuenses in the section Sorbomalus are

intermediate in evolution. In general, Southwest China is regarded as the probable

area of the origin of Malus, and the section Docyniopsis and the series

Yunnanenses as the relict primitive forms. This area is probably also the center

of differentiation.

A good deal of attention has been paid by horticulturists. A comparison of

the karyotypes in the series Pumilae shows that the diploid M. sieversii has 14 m

chromosomes, while the other diploids M. pumila, M. sp. and an alien

species M. sylvestris Mill. from Europe ( Liang, unpub.) have 12 m

chromosomes. Thus M. sieversii may be relatively primitive, although the ratio

of the longest to the shortest and the ratio of mean arm ratio are similar. The

present investigation confirms Ponomarenko’s view that the diploid M. sieversii

might be an ancestor of cultivated apple (M. pumila).

The wild polyploids of Malus were usually considered as allopolyploid

(Brown, 1976). Studies on the meiotic pairing of five triploids and one tetraploid

have revealed diverse types of polyploidy. M. hupehensis is a typical allotriploid

species, M. spectabilis, M. sieboldii, M. toringoides and M. transitoria are

segmental allotriploids, and M. xiaojinensis is almost an autotetraploid.

Apomixis in all known polyploid species of Malus is facultative. The princi-

pal adaptive advantage of facultative apomixis is to make sterile individuals

restore fertility, and also to maintain evolutionary potential. Therefore, it plays

an important role in evolution of Malus, but taxonomy of apomictical groups re-

mains highly controversial. Usually, the apomictical taxa are not treated as

species level (Stace, 1980; Hong, 1990). In Rehder’s (1940) system, only

M. hupehensis was recognized as species in apomictical taxa of Malus. We think

this treatment to be reasonable because M. hupehensis is an allotripoid and so far

its diplois form has not been found. New species M. xiaojinensis described

by Cheng and Jiang (1985) is nearly an autotetraploid, and its karyotype consti-

tution is extremely similar to the tetraploid form of M. toringoides from Aba ,

Sichuan Province, especially the remarkable similarities of the smallest deleted

chromosome, morphology and biochemistry (Li, 1990). We think, therefor, that

the triploid form of M. toringoides from Aba, Sichuan with 3x, 4x mixploidy

might be the maternal plant of M. xiaojinensis. For this reason, M. xiaojinensis

should be placed in apomictical polyploid complex of M. toringoides of the series

Kansuensis, and this is also in accordance with Rehder’s classfication.

Key words: Karyotype, Cytotaxonomy, Malus, China

Liang Guo-lu, Li Xiao-lin. Chromosome Studies of Chinese Species of Malus Mill.[J]. .

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Chromosome Studies of Chinese Species of Malus Mill.

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