Cytology and leafspot resistance in Arachis hypogaea x wild species hybrids

Euphytica 3 I (1982) 885-893

CYTOLOGY AND LEAFSPOT RESISTANCE IN ARACHIS HYPOGAEA x WILD SPECIES HYBRIDS

M. COMPANY, H. T. STALKER and J. C. WYNNE

Dept. ofCrop Science, N. C. State University, Raleigh, North Carolina 27650, USA

Received9 February 1982

INDEX WORDS

Arachis hypogaea, groundnut, peanut, interspecific hybrids, Cercospora arachidicola, leafspot

SUMMARY

Introgression of germplasm from diploid wild Arachis species to A. hypogaea has great potential for improv- ing pest resistance in cultivated peanuts. This investigation evaluated methods for incorporating exotic germplasm into cultivated peanuts, especially for Cercospora arachidicola HORI resistance. Interspecific hybrids between A. hypogaea (cvs. NC 2 and NC 5) and the wild species A. cardenasii KRAP. et GREG. nom. nud. and A. chacoense KRAP. et GREG. nom. nud. were analyzed cytologically and for leafspot resistance. All Fi hybrids were sterile, had irregular meiosis, and very few multivalents. They were highly resistant to C. arachidicola in field tests and had a IO-fold reduction of conidia per lesion in the greenhouse as compared to A. hypogaea cuhivars. After colchicine treatments of Fi hybrids, hexaploids (28 = 60) and aneuploids (2n = 54, 56, 63) were observed. The hexaploids had up to 18 univalents per pollen mother cell and very few multivalents, indicating a low frequency of intergenomic chromosome pairing. For C. arachidicola resistance, significant differences were not found among wild species parents, Fi hybrids and two generations of hexaploids. Most hexaploids were stable at 2n = 60 and embryos aborted when back- crosses with the respective wild species were attempted. However, when hexaploids were backcrossed to A. hypogaea, several fertile pentaploid (2n = 50) offspring were obtained. Use of self-pollinating pentaploids is believed to be the quickest method to recover 40-chromosome hybrid derivatives in these hybrids.

INTRODUCTION

The cultivated peanut, Am&s hypogaea L. (2n = 40) is an allotetraploid species native to South America. Improvement of cultivars has been attempted with adapted and exotic collections of A. hypogaea and with wild Arachis species (2n = 20, 40). The wild species are potentially of great value for incorporating high levels of disease and insect resistances into the cultivated species. Although the species belonging to section Arachis will hybridize with A. hypogueu, sterility - in large part due to ploidy level differences - has restricted germplasm introgression to the cultivated species.

KRAPOVICKAS & RIGONI (1951) reported the first successful interspecific hybrid between A. hypogueu and a wild diploid species, A. villosu var. correntinu BURK. Since then interspecific hybrids have been made between the cultivated peanut and at least 10 species of section Aruchis (see SMARTT & GREGORY, 1967; RAMAN, 1976; GREGORY

’ Paper number 8114 of the Journal series of the North Carolina Agricultural Research Service, Raleigh, NC 27650. The investigation was supported by SEA-CR Research Agreement 5901-410-9-347 and was submitted by the senior author as partial fulfillment of the requirements for the Master of Science degree in Crop Science.

885

M. COMPANY, H. T. STALKER AND I. C. WYNNE

& GREGORY, 1979). Cytologically most hybrids had chromosome associations of 10 I + 10 II with trivalents occasionally present during the metaphase stage of meiosis (KUMAR et al., 1957; RAMAN, 1959, 1976; D'CRUZ & UPADHYAYA, 1962; SMARTT, 1965). However, SMARTT (1965) also reported an average of 2.5 III (range O-5) per cell in A. duranensis KRAP. et GREG. nom. nud. hybrids and 3.4 III (range +6) per cell in A. heloides MART. ex KRAP. et RIGONI hybrids. Moss et al. (1981) reported at least half of the pollen mother cells of A. chacoense KRAP. et GREG. nom. nud. or A. stenosperma KRAP. et GREG. nom. nud. hybrids with A. hypogaea had a trivalent.

Fertility has been restored after colchicine treatments or by natural chromosome doubling in at least seven interspecific A. hypogaea hybrid combinations, including those with the following species: A. villosa BURK, A. villosa var. correntina, A. cardenasii KRAP. et GREG. nom. nud., A. duranensis, A. stenosperma and A. batizocoi KRAP. et GREG.(KUMAR etal.,1957; RAMAN, 1960; D'CRUZ& CHAKRAVARTY, 1960,1961; SMARTT & GREGORY, 1967; KRAPOVICKAS etal., 1974; SPIELMAN et al., 1979). A high frequency of univalents was reported in hexaploid hybrids with A. villosa var. correntina (D'CRUZ&CHAKRAVARTY, 1961)and A.cardenasii (SPIELMAN etal. ,I 979). KRAP- OVICKAS et al. (1974) also reported interspecific hybrids with A. batizocoi had 40 chromosomes, and DAVIS & SIMPSON (1976) reported plants ranging from 32 to 48 chromosomes in A. cardenasii hybrid derivatives. Furthermore, STALKER et al. (1979) reported 40-chromosome selections from the same A. cardenasii population with good agro- nomic characters; however, the pathway from 60 to 40 chromosomes, whether the result of outcrossing or somatic chromosome loss, is unknown.

In the North Carolina-Virginia peanut production area several insects and diseases attack peanuts. The pest with greatest economic impact is Cercospora arachidicola HORI where severe defoliation results when fungicides are not applied to the foliage. Among the species which will hybridize with A. hypogaea, three have very high levels of resistance to C. arachidicola including A. chacoense, A. cardenasii and A. stenosperma (ABDOU et al., 1974; Moss, 1977; KOLAWOLE, 1976; FOSTER et al., 1981). Moss (1980) reported interspecific hybrids between A. hypogaea and the above three species were resistant to C. arachidicola when evaluated in Malawi.

The purpose of this investigation was to determine efficient pathways for introgress- ing germplasm from diploid Arachis species to A, hypogaea and to evaluate interspecific hybrids for C. arachidicola resistance.

MATERIALSAND METHODS

The parental material used in this investigation included the A. hypogaea cultivars, NC 2 and NC 5, and two diploid wild species of section Arachis, A. cardenasii and A. chacoense. Triploid Fi interspecific hybrids were produced and colchicine treated to restore fertility. Only hexaploids which expressed female fertility were studied and the remaining sterile plants were discarded. The fertile hexaploid plants were self-pollinated and the chromosome numbers of second generation hexaploids were deter- mined. Meiotic behavior in the pollen mother cells of parental materials, Fi hybrids and the first generation hexaploids was analyzed. Flower buds were fixed in Carnoy’s solution (6 absolute alcohol:3 chloroform:1 glacial acetic acid), macerated and squashed in acetocarmine. Late diakinesis or early metaphase I stages of meiosis were

886 Euphyrica 31 (1982)

LEAF SPOT RESISTANCE IN ARACHIS

then observed. For mitotic chromosome counts, root tips were pretreated in a saturat- ed solution of paradichlorobenzene for 30 min, hydrolyzed in 9 pt-45% glacial acetic acid: 1 pt-I N HCl, macerated, pressed in an oil press at 211 kg/cm2 and stained with aceto-orceine. Pollen viability was estimated by the acetocarmine-glycerine technique of PITTENGER & FROLICK( 1951).

Parents and all hybrid generations were also evaluated for resistance to C. arachidicola in field and greenhouse experiments. Resistance to natural infection was evaluated in four replicates of a randomized complete block field design at Lewiston, NC during 1979 and 1980. Five leaves per plant were scored for the number of lesions per leaf and for defoliation, measured as a percentage of leaves fallen from plants. In the greenhouse, leaves of cuttings were inoculated with lo4 conidia/ml of C. arachidicofa. The test was replicated twice, with three cuttings per plot, and inoculated plants were incubated for 3 weeks in a mist chamber. Six lesions were removed from leaves, incubated on a moist filter paper for 2 days, transferred to a vial with an emulsion of Tween-80 in water and the number of conidia per lesion estimated (FOSTER et al., 1980).

RESULTS

Hybrids were produced between the A. hypogaea cultivars NC 2 and NC 5 and the diploid species A. cardenasii and A. chacoense. Reciprocal differences were observed in the success rate of hybrid production where crosses are more successful when A. hypogaea is used as the female parent (Table 1). Subsequent analyses were only made on hybrids in which A. hypogaea was used as the female parent. Cytologically, the wild species had 10 II and normal meiosis while the A. hypogaea cultivars had mostly 20 II, but univalents were also observed (Table 2). Triploid interspecific hybrids had 10 I + 10 II with occasional trivalents.

Stem cuttings were colchicine-treated after which 15 NC 5 x A. cardenasii, 10 NC 2 x A. chacoense and 30 NC 5 x A. chacoense hybrids were produced. Although 55 total hexaploids were isolated, only 24 were female fertile and plants failing to produce seeds were discarded. Most of the plants had 60 chromosomes, but aneuploids with 2n = 54, 56 and 63 were also observed. During meiosis the A. cardenasii hybrids had up to 13 univalents and the 60-chromosome A. chacoense hybrids had a maximum of 18 univalents (Table 2). The average frequency of multivalents was less than one per pollen mother cell and only trivalents or quadrivalents were observed. A similar

Table 1. Interspecific hybrids produced between cultivated and wild Arachis species.

Hybrid Number

pollinations hybrids

% Success

A. hypogaea x A. curdenasii 388 14 3.6 A. cardenasii x A. hypogaea 565 1 0.2

A. hypogaea x A. chacoense 396 109 21.5 A. chacoense x A. hypogaea 516 4 0.8

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M.COMPANY,H.T.STALKERANDJ.C.WYNNE

Table 2. Chromosome associations and ranges for parents, triploid interspecific hybrids and their hexaploid derivatives.

Parent/hybrid 2n Number of Chromosome association (avg and range)

plants cells I II III IV

A. cardenasii 20 4 100 A. chacoense 20 4 100 NC2 40 2 72

10 10 19.9 (19-20) 19.9 (19-20)

;AO) 10.9 (9-13) 9.9 (8-12) 27.1 (22-30) 26.2 (18-29) 26.3 (21-29) 25.0 (23-26) 21.1 (18-25) 25.8 (I 7-30)

0.6 P-2) 0.2 (o-2) 9.9 (9-12) 8.1 (4-12) 9.9 (6-14) 4.3 (O-13) 5.4 (2-14) 5.7 (2-18) 4.0 G-8) 12.2 (6-W 7.2 (2-20)

74 NC5 40 2

NC 5 x A. cardenasii

NC 2 x A. chacoense

30

30

30

1 50 0.1 Pl) 0.4 (O-1) 0.1 (O-1) 0.2 0.3 (o-3) (O-3) 0.2 0.3 (o-2) W5) 0.2 0.3 (O-2) (@5) 0.1 0.1 (O-1) (O-1) 0.4 0.1 (o-2) P-1) 0.7 0.5 (o-3) W3)

1 28

NC 5 x A. chacoense 25

NC 5 x A. cardenasii 60


117

93

NC 5 x A. chacoense 60 104


56

63

26

1 29

I 26

pattern of many univalents and few multivalents was observed for the three aneuploid plants (Table 2).

Hexaploid plants were self-pollinated in an attempt to produce offspring with fewer than 60 chromosomes. Chromosome numbers in 134 offspring derived from 2n = 60 plants were counted. One hundred seventeen offspring had 2n = 60 while the remaining 17 progeny ranged from 2n = 58 to 62 (Table 3). The ploidy level thus ap- peared to remain near 2n = 60 even though the parental hexaploids had irregular meiotic divisions. Although the two offspring plants of the 2n = 54 chromosome hybrid had 43 and 53 chromosomes, respectively, the plant at the lower chromosome number died during the early seedling stage.

Two hybridization programs were then initiated to attempt lowering the chromosome number of interspecific hybrids to the same ploidy level as A. hypogaea. Hexa- ploids were backcrossed with the respective wild species parent, but after 655 pollinations no viable seeds were recovered. Although many pods developed normally, embryos aborted during early developmental stages. The hexaploids were also backcrossed to A. hypogaea cultivars NC 2, NC 5, NC 6 and Florigiant. The success rate of backcrossing was highly dependent on the A. hypogueu parent used in the cross,

888 Euphytica 31 (1982)


Table 3. Distribution of chromosome numbers in progeny from self-pollinated hexaploids.

Hybrid 2n Progeny chromosome numbers

43 53 54 56 58 59 60 61 62

NC 5 x A. cardenasii-I 60 NC 5 x A. cardenasii-2 60 NC 5 x A. cardenasii-3 60 NC 2 x A. chacoense-1 60 NC 2 x A. chacoense-2 60 NC 5 x A. chacoense-1 56

60 NC 5 x A. chacoense-2 54

60

I 3 27

1 1 2 19 1 3 I 20 2

1 2 3 11 1 9 1

II 3 2 34

I 1 3 1 7 11 1282 2

Table 4. Results of hybridization program with A. hypogaea cultivars and hexaploid interspecific hybrids and resulting pentaploids.

Cross Polli- Penta- Avg Self-pollinated nations ploids pollen pentaploid seeds

stained number 2n

NC 5 x 6x (NC 5 x A. cardenasii) NC 6 x 6x (NC 5 x A. cardenasii) Florigiant x 6x (NC 5 x A. cardenasii) NC 2 x 6x (NC 2 x A. chacoense) NC 6 x 6.x (NC 2 x A. chacoense) Florigiant x 6x (NC 2 x A. chacoense) NC 5 x 6x (NC 5 x A. chacoense) NC 6 x 6x (NC 5 x A. chacoense)

251 1 95.0 1 50 287 10 84.6 18 45-50 106 0 - - -

137 2 88.6 1 46 252 1 87.3 3 50 102 II 70.7 0 - 206 0 - 0 - 174 3 - 0 -

1521 28 19.3 23

but 28 pentaploids, representing both A. cardenasii and A. chacoense hybrids, were produced after 1521 pollinations (Table 4). Although the plants were vegetatively vig- orous, most of the 2n = 50 plants only produced zero to five flowers per day during the summer months in North Carolina and meiosis could not be observed. Based on pollen stained with aceto-carmine, the pentaploid hybrids averaged 79.3% male fertility (range 70 to 95%). Furthermore, after self-fertilization the pentaploids produced 23 seeds which ranged in chromosome number from 2n = 45 to 50 (Table 4). These pentaploid progenies were highly variable in leaf shapes, plant size, number of branches, number of flowers produced. Most plants produced seeds when self-pollinated. During the summer of 1980,764 pollinations between A. hypogaea and the pentaploid hybrids did not result in hybrids. Hence, the production of many pentaploid hybrid derivatives, and self-pollinating these plants to lose chromosomes, may be the easiest method to obtain 40-chromosome plants.

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M. COMPANY, H. T. STALKER AND J. C. WYNNE

Table 5. Means for number of Cercospora arachidicola lesions on Arachis species and hybrids at Lewiston, NC.

Species/hybrid

A. cardenasii A. chacoense NC2 NC5 Florigiant NC 3033 NC 5 x A. cardenasii

NC 2 x A. chacoense

NC 5 x A. chacoense

Genera- tion’

F1

Cl

c2

FI Cl

c2

Fl Cl

c2

Number of genotypes

8

8

1 5

5

1 11

11

Avg lesion number/5 leaves2

1979 1980

0.47a 0.13 a 0.27 a 0.00 a

41.23 c 27.73 c 17.82 b 10.20 b 54.36 d 34.20 d 33.81 c 14.20 b

1.77 a 1.02 a (0.80-2.73) (0.76-1.13)

3.74 a 1.08 a (1.12-9.27) (0.75-2.33)

1.51 a 0.23 a (0.37-2.89) (0.061.53)

6.57 ab 1.53 a 6.14 ab 1.38 a

(3.1 lH.30) (0.95-I .69) 2.52 a 2.06 a

(1.56-4.25) (0.60-3.05) 0.73 a 0.53 a 4.01 a 0.91 a

(0.0-9.51) (0.13-1.33) 2.47 a 1.00 a

(0.433544) (0.62.36)

’ Ft hybrids have 2n = 30, Ct and C2 hybrids have 2n = 60 and are the first and second generation hexaploids, respectively. 2 Means followed by same letters are not signiticantly different at p = 0.05. Numbers in parentheses repre- sent ranges.

In addition to cytologically analyzing interspecific hybrid derivatives, the plant materials in this investigation were rated for resistance to C. arachidicolu. The wild species A. cardenasii and A. chacoense averaged only 0.30 and 0.14 lesions per five leaves, respectively. The triploid or hexaploid interspecific hybrids with either A. cardenusii or A. chacoense in their background were not significantly different from the wild species, although a range of 1.1 to 9.3 lesions per five leaves for A. cardenasii hexaploid hybrids and 0 to 9.5 lesions for A. chacoense hybrids were observed (Table 5). Hybrids with NC 2 consistently had more lesions per leaf than hybrids with NC 5. An analysis of variance among genotypes and hybrid generations revealed that all hybrid derivatives were nonsignificantly different from each other and from the wild species but significantly different (p = 0.01) from the A. hypoguea cultivars.

To evaluate possible mechanisms for resistance to C. arachidicola, conidia production and lesion size on detached peanut leaves were analyzed in the greenhouse. The correlation coefficient between conidia number per lesion and number of conidia per mm2 oflesion was r = 0.51 which was not significant at p = 0.05. Triploid interspecific hybrids had significantly fewer (p = 0.01) lesions than the A. hypogaea cultivars (Table

890 Euphytica 3 I (1982)


Table 6. Means for cultivars and interspecific Ft hybrids for Cercospora arachidicola conidia production.

Parent/hybrid Number of conidia Number of conidia per lesion’ mm2 of lesion

A. cardenasii -

A. chacoense -

NC 5 x A. cardenasii-1 23a NC 5 x A. cardenasii-2 97a NC 2 x A. chacoense-I 185a NC 5 x A. chacoense-2 166a NC2 2652b NC5 2195b Florigiant 3901b NC 3033 2430b

- -

31a 30a 80a

207a 422b

1014b 495b 496b

’ Means followed by the same letter are not significantly different at p = 0.05. 2 A. cardenasii and A. chacoense did not produce lesions after inoculation with C. arachidicola conidia.

6). In general, A. cardenasii hybrids had fewer conidia per lesion than A. chacoense hybrids.

DISCUSSION

Several wild diploid species of section Arachis have high levels of resistance to one or more disease or insect pests of peanuts. Exploitation of this germplasm resource has been slow, in large part because fertility is difficult to restore in interspecific hybrids. Sterility barriers in the form of nonflowering or nonpegging plants are severe at several ploidy levels and creation of 40-chromosome populations is a difficult pro- cess. Although hexaploid plants of A. hypogaea x A. cardenasii or A. hypogaea x A. chacoense can be produced after colchicine treating stem cuttings, many of the plants are completely sterile. The fertile plants are chromosomally stable at the hexaploid level even though meiosis is highly irregular. Because of the high frequency of univalents and corresponding low frequency of multivalents, intergenomic gene transfer between wild and cultivated species is unlikely at high ploidy levels in interspecific Arachis hybrids.

The most tenable pathway from 60 to 40-chromosome plants is to backcross polyp- loid hybrids with the cultivated parent after which 50-chromosome intermediates will be produced. The quicker route of hybridizing hexaploids with the respective diploid wild species failed in North Carolina. Pentaploids have some degree of fertility and at least a few individual plants have produced offspring. Although most progeny re- mained at 2n = 50, chromosome numbers to 2n = 45 were observed. A second backcross generation would seem desirable so that 2n = 40 offspring could more quickly be produced, but attempts again failed. Self-pollinating pentaploids and then selecting aneuploid offspring is believed to be the most tenable method to produce 40-chromosome hybrid derivatives in A. cardenasii and A. chacoense hybrids.

The underlying reason for producing interspecific hybrids was to transfer C. arachidicofa resistance from the wild to cultivated species. This investigation indicated that

Euphytica 31 (1982) 891

M.COMPANY,H.T.STALKERAND J.C.WYNNE

C. arachidicola resistance behaves like a dominant trait in interspecific hybrids where high levels of resistance were observed in all genotypes. Furthermore, a study of conidia production indicated that the resistance is genetically controlled and not a function of seed set or rapid vegetative growth. Although variation in numbers of lesions per leaf was observed among hexaploid plants, differences were not detected among generations. This indicates that a breeding program at the hexaploid level is unwarranted; selection of resistant first-generation hexaploid plants should be made and immediate- ly backcrossed to the cultivated parent. Selection for desirable traits should then be made in populations of 40-chromosome hybrid derivatives.

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ABDOU, Y. A. M., W. C. GREGORY & W. E. COOPER, 1974. Sources and nature of resistance to Cercosporu arachidicola HORI. and Cercosporidiumpersonatum (BECK. et CURTIS) DEIGHTON in Arachis species. Pea- nut Sci. 1:6-l 1.

DAVIS, K. S. & C. E. SIMPSON, 1976. Variable chromosome numbers in two ‘amphidiploid’ populations of Arachis. Proc. Am. Peanut Res. Educ. Assoc. 8:93.

D’CRUZ, R. & K. CHAKRAVARN, 1960. A case of asynapsis in Arachis allopolyploid. Poona Agric. Coll. May. 5 1:15-l 7 (cited in PI. Br. Abstr. 3 1 (1961): 3803).

D’CRUZ, R. & K. CHAKRAVARTY, 1961. Spontaneous allopolyploidy in Arachis. Indian Oilseeds J. 5: 55-57. D’CRLJZ, R. & B. R. UPADHYAYA, 1962. Allopolyploidy in Arachis. Indian Oilseeds J. 6: 33-37. FOSTER, D. J., H. T. STALKER, J. C. WYNNE & M. K. BEUTE, 1981. Resistance of Arachis hypogaea and

wild relatives to Cercospora arachidicola HORI. Oleagineux 3: 139-143. FOSTER, D. J., J. C. WYNNE & M. K. BEUTE, 1980. Evaluation of detached leaf culture for screening peanuts

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pecitico es&i1 de Arachis (Leguminosae). Bonplandia 3: 129-142. KRAPOVICKAS, A. &V. A. RIGONI, 1951. Estudios citologicas en el genero Arachis. Rev. Invest. Agr. (Buenos

Aires) 5: 289-293. KUMAR, L. S. S., R. D’CRUZ & J. G. OKE, 1957. A synthetic allohexaploid in Arachis. Curr. Sci. 26:

121-122. Moss, J. P., 1977. Cercospora resistance of interspecific Arachis hybrids. Proc. Amer. Peanut Res. Educ.

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Moss, J. P., I. V. SPIELMAN, A. P. BURGE, A. K. SINGH & R. W. GIBBONS, 198 1. Utilisation of wild Arachis species as a source ofcercospora leafspot resistance in groundnut breeding, pp. 673-677. In: G. K. MANNA & U. SINHA (Eds), Proc. 3rd All India Congr. Cytogenetics. Hindasia Publ., Delhi.

PITTENGER, T. H. & E. F. FROLIK, 1951. Temporary mounts for pollen abortion determination. Stain Tech. 26: 181-184.

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SMARTT, J. & W. C. GREGORY, 1967. Interspecific cross-compatibility between the cultivated peanut Arachis hypogaea L. and other members of the genus Arachis. Oleagineux 22: 455-459.

SPIELMAN, I. V., A. P. BURCE & J. P. Moss, 1979. Chromosome loss and meiotic behaviour in interspecific hybrids in the genus Arachis L. and their implications in breeding for disease resistance. Z. Pflanzenziich- tung 83: 236250.

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Cytology and leafspot resistance in Arachis hypogaea x wild species hybrids