Hybrid Sterility

Cytogenetic Differentiation into Western and Eastern Geographical Groups

Thirty-five accessions derived from the entire distribution area were crossed with the four Tester lines whose F1 hybrids were highly sterile (Tanaka et al. 1967; Ohta 1992). Pollen fertility in the F1 hybrids obtained from a total of 134 cross combinations varied from 0 % to 96.3 % (Table 6.1), and it was significantly correlated with their seed fertility by open-pollination (r = 0.786). The fertility in the F1 hybrids was correlated with the geographical origin of the parental accessions. Based on the

Table 6.1 Pollen fertility (%) in the F1 hybrids between the four Testers and the 35 accessions of

Ae. caudata from its entire distribution area (Ohta 2000)

Region

Site no.

Accession no. (KU)

Varietya

Tester Ab

Tester B

Tester C

Tester D

Aegean Islands

1

12044B

P

0

0

0

25.0

2

12055B

P

0

0

14.5

83.5

3

12155B

P

0

0

0

69.1

4

12162A

T

0

0

0

72.2

5

12163A

T

0.1

0

0.8

96.3

6

12073A

T

0

0.1

0

0

7

12100B

P

0

15.1

42.6

86.3

8

12177B

P

0

0.1

8.9

56.0

9

12121A

T

0

0.3

0.2

0.1

West Anatolia

10

11401

T

0

0

13.7

68.8

11

11403

T

0

No obs

22.7

8.9

12

12169

T

0.8

0.2

20.3

36.9

Central Anatolia

13

5853

P

0

17.8

0

No obs

14

5854

P

No obs

51.8

0

0

15

5859

P

No obs

65.4

0

0

16

5860

P

13.1

0

0

0

17

5863

P

0.1

31.3

0

0

18

5888

P

0.2

95.5

0

0

19

5896

P

No obs

66.9

0

0

20

12165

P

10.3

1.5

0

0

South Anatolia

21

5867

P

14.1

28.9

0

0

22

5897

P

62.5

0

No obs

No obs

23

5899

P

93.3

0.5

0

3.5

East Anatolia

24

5484

P

24.2

0

0

0

25

5485

T

0

61.0

0

0

26

5489

T

27.0

19.3

0

0

27

5490

P

69.7

9.4

0

0

28

5491

P

95.3

17.2

0

0

29

5492

P

75.4

19.1

0

0

30

5885

T

65.9

11.9

0

0

31

5889

P

81.3

0.4

0

0

32

5891

T

7.5

54.3

0

0

Northern Iraq

33

5475

P

72.8

43.0

0

0

34

5472

T

66.2

6.0

0

0.2

35

5482

P

34.0

4.1

0

0

aP, var. polyathera; T, var. typica

bTester A, KU6-2, var. typica from Syria; Tester B, KU5852, var. polyathera from Central Anatolia; Tester C, KU5864, var. typica from West Anatolia; Tester D, KU5871, var. typica from Greece

Fig. 6.1 Collection sites of the four Testers (A, B, C and D) and 35 accessions of Ae. caudata used in the present study (Ohta 2000). Mountainous area higher than 1,500 m above sea level is shadowed. A broken line indicates the border between the western and eastern regions defined from hybrid sterility

fertility in the F1 hybrids, the 39 accessions including the four Testers could be grouped into the western and eastern geographical groups: the former consists of the accessions from the Greek mainland, the Aegean Islands and West Anatolia, while the latter from Central Anatolia, South Anatolia, East Anatolia, northern Syria and Iraq. The two geographical regions are isolated with the mountains lying between West Anatolia and Central Anatolia (Fig. 6.1).

Gametocidal-Like Genes Cause the Sterility in Intraspecific F1 Hybrids

The highly sterile F1 hybrids from the two cross combinations between the parental lines derived from the western and eastern regions were backcrossed to their parental lines. The BC1F1 plants distinctly restored their fertility when the F1 hybrids were backcrossed to the western parents, while their fertility was not restored when they were backcrossed to the eastern parents (Table 6.2). A similar sterility has been reported in several Aegilops species as the phenomenon that certain Aegilops chromosomes, gametocidal chromosomes, were preferentially transmitted in the genetic background of common wheat (Endo and Tsunewaki 1975; Maan 1975; Endo 1990). An Ae. caudata chromosome was found to be selectively retained in common wheat (Endo and Katayama 1978). Monosomic addition plants for a gametocidal chromosome to common wheat showed semi-sterility because only gametophytes with a gametocidal chromosome are functional while those without this chromosome are aborted. As a result, gametocidal chromosomes were preferentially transmitted to the next generation. When normal common wheat plants with

Table 6.2 Frequency distribution of the pollen fertility in the BC1F1 populations obtained from back-crossing the sterile F1 hybrids between the western and eastern accessions to their parental lines

Cross group

F1 hybrid origin

Pollen fertility (%) in F1

Parental line backcrossed

Total no. of BC1F1 plants

Pollen fertility (%) class and no. of BC1F1 plants observed

<10

<20

<30

<40

<50

<60

<70

<80

<90

<100

Eastern acc.× Western testers

KU5859 × C

0

KU5859

2

2

KU5859 × D

0

KU5859

1

1

D

1

1

Western acc.× Eastern testers

KU12169 ×A

2.4

A

7

3

2

2

KU12169

6

5

1

KU12169 × B

7.7

B

1

1

KU12169

3

3

Fig. 6.2 A schema of the present gametocidal gene hypothesis explaining hybrid sterility in the F1 hybrids between the western and eastern accessions and its restoration in the BC1F1 plants obtained from backcrosses to the western parental lines

42 chromosomes were crossed to the monosomic addition plants, the obtained plants had 43 chromosomes (monosomic addition for gametocidal chromosome) and showed semi-sterility again. Disomic addition plants for gametocidal chromosomes showed normal fertility, and such chromosomes were transmitted in stable by self-pollination.

In the present study, a similar phenomenon as that caused by gametocidal chromosomes in wheat genetic background was observed in the F1 and BC1F1 plants of Ae. caudata with normal chromosome constitution, 2n = 14. This result strongly suggests that the sterility in the F1 hybrids between the western and eastern accessions is caused not by gametocidal chromosomes but by certain gene(s) on gametocidal chromosomes, and that gametocidal gene(s) cause sterility and their preferential transmission in heterozygotes not only in the genetic background of common wheat but also in the normal genetic background in Ae. caudata. A schema of the present gametocidal gene hypothesis explaining hybrid sterility is shown in Fig. 6.2, where we propose that the western and eastern accessions carry dominant and recessive gametocidal alleles, respectively.

 
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