Annals of Botany 89: 31-39, 2002
© 2002 Annals of Botany Company
Partial Hybridization in Wide Crosses between Cultivated Sunflower and the Perennial Helianthus Species H. mollis and H. orgyalis
1INRA, UR-GAP Bâtiment 33, UMR 1097 Diversité et Génome des Plantes Cultivées, 2 Place Viala, F-34060 Montpellier cedex 1, France
* For correspondence. Fax +33 4 67 04 54 15, e-mail berville{at}ensam.inra.fr
Received: 10 April 2001; Returned for revision: 14 June 2001; Accepted: 14 September 2001.
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ABSTRACT |
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To obtain introgressed sunflower lines with improved disease resistance, interspecific crosses were performed with foreign perennial species. We report on several unusual features displayed by these hybrid plants. The methods used to produce the kernels affected yield and genotypes of progeny. Phenotypic traits and DNA markers were investigated in 97 plants derived from cross-pollination between annual diploid cultivated sunflower (Helianthus annuus) and the perennial diploid species H. mollis or H. orgyalis, and the reverse reciprocal crosses. The level of hybridization in progeny was determined using RAPD and RFLP markers. Hybridization was performed by leaving embryos to develop normally on the head (classical crossing) or using embryo rescue. All observed plants derived from H. mollis were diploid (2n = 34). Phenotypes were predominantly similar to the female when cultivated sunflower was the female parent. Progeny from crosses using a wild species as the female parent resembled that parent. Thus, reciprocal crosses led to different progeny. F1 sister progeny shared different sets of molecular markers representing a few of those of the wild species used as the pollen donor. Our results indicate mechanisms leading to the unusual event of partial hybridization. Possible mechanisms behind these unusual events and their possible impact on evolution are discussed.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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To improve sunflower, desirable genes are sought from related wild Helianthus species. Some of these species carry relevant traits such as disease resistance (downy mildew, Phomopsis, Rust, Sclerotinia), cytoplasmic male sterility (CMS), male-fertility restoration (Rf), abiotic stress tolerance (drought, salinity), or architecture (petiole-less type). However, breeders who have performed interspecific crosses have obtained results that are difficult to explain. Therefore, we carried out interspecific crosses with two wild Helianthus species that displayed different F1 progeny types depending on the method of crossing used.
Section Helianthus of Helianthus L. includes cultivated sunflower and ten other annual species; all are diploid (2n = 34) (Schilling and Heiser, 1981). Crosses between cultivated sunflower and these species have been performed without difficulty, but the resulting progeny are more or less female sterile due to translocations (Chandler et al., 1986). In such crosses performed for breeding purposes, cultivated sunflower is usually used as the female parent to avoid loss of the cytoplasm, unless the cytoplasm from the wild species is desired (Serieys, 1984).
Section Atrorubentes includes perennial diploid species such as H. mollis and hexaploid species such as Jerusalem artichoke (H. tuberosus L.). Russian breeders have also named some species, supposedly natural hybrids that occurred under field collections (Anashenko, pers. comm.). These have not been recognized by Schilling and Heiser (1981). Helianthus orgyalis, a perennial diploid used in the present study, corresponds to one of these species and not to the H. orgyalis Wats. recognized by Watson (1929) as a synonym of H. salicifolius.
Annual and perennial species have been found to differ in their genomic constitution (Sossey-Alaoui et al., 1998). Sunflower and all annual species in Section Helianthus carry two basic genomes, H and C. Perennial species including H. mollis and H. orgyalis of Section Atrorubentes carry the C, P and A genomes. Helianthus appears to be a segmental allopolyploid or palaeopolyploid genus (Wendel, 2000). This genomic classification agrees with the phenotypic classification of Schilling and Heiser (1981).
Wide crosses between different species of Helianthus, i.e. annual x perennial species, have been reported in the literature. These crosses were obtained either by pollination and natural achene development or by use of in vitro embryo rescue methods (Georgieva-Todorova, 1984; Christov, 1991; Cazaux et al., 1996; Jan, 1996; Korell et al., 1996; Natali et al., 1998; Sukno et al., 1998; Faure et al., 2000). In crosses between sunflower and wild annuals, progeny were diploid (2n = 34). Crosses between sunflower and perennial species, e.g. pollinating sunflower with hexaploid (Jerusalem artichoke) or diploid species (H. mollis or H. maximiliani) of Section Atrorubentes, usually failed. However, some achenes were recovered which displayed hybrid traits. When sunflower was crossed with Jerusalem artichoke, F1 hybrid plants carried 51 chromosomes and showed reduced pollen fertility. In the past, perennial species have been crossed with sunflower and most of the improvements in sunflower have been the result of crossing with Jerusalem artichoke (Leclercq et al., 1970; Pustovoit et al., 1976). For crosses involving H. mollis or other diploid species the situation is much less clear. Apparently incompatible results have been reported concerning the hybridization process leading to development of achenes (Krauter et al., 1991; Faure et al., 2000).
We produced progeny either by natural embryo development or using embryo rescue techniques. Progeny was examined for phenotypic traits and was genotyped for markers diagnostic of the parents. This enabled us to the determine the status of each plant and whether it was a hybrid, and also to quantify the level of hybridization occurring in such crosses. In addition, comparison of hybrid traits and molecular markers showed that this material was of potential value to breeders. The impact of such a mechanism on evolution is considered and mechanisms underlying the level of partial hybridization are discussed.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Crossing techniques
Pollination of cultivated sunflower by perennial species.
Fifteen interspecific pollination experiments were performed in the field during the summers of 1994 (crosses 1–11) and 1997 (crosses 12 and 13). Each experiment was carried out on flower heads which had been protected from foreign pollen by bagging. Twelve crosses (1–12) involved CMS-PET1 (H. petiolaris cyroplasm) male-sterile cultivated sunflower lines 85A3, FT2603, HA89 or (HA89 x AA724) F1 hybrid. These cultivated genotypes were pollinated by H. mollis accession number 230 (from INRA collection = PI 435749) or accessions 600 and 742 (PI 468761). The reciprocal cross with H. mollis as female was performed in crosses 14 and 15. One experiment (cross 13) used CMS-PET1 female sunflower x H. orgyalis accession number 108 from INRA collection (received from VIR, Russia).
Mature achenes from interspecific pollinations 1–11 (1994) were harvested from female sunflowers and were germinated in Petri dishes. Seedlings were potted and transferred to a glasshouse after 1 week. In 1997, 7-d-old excised embryos from crosses 12 and 13 were transplanted in vitro onto MS-modified growth medium (Alissa et al., 1986) to induce their development (embryo rescue). All seedlings and plants were grown in a glasshouse under a controlled photo- (16 h day) and thermoperiod (day 25 °C, night 18 °C).
Pollination of perennial H. mollis by cultivated sunflower.
In 1994, Helianthus mollis accessions 742 (derived from PI 468761; cross 14) and 286 (derived from VIR-Russia; cross 15) were pollinated by cultivated sunflower inbred lines LA, WG, HA89 or RHA274 to increase the chances of crossing. Self-fertilization did not occur due to self-incompatibility in H. mollis 742 and cytoplasmic male sterility in H. mollis 286. Mature achenes were obtained from crosses 14 and 15 and were treated as described previously before being transferred to a glasshouse with a controlled photo- (16 h day) and thermoperiod (day 25 °C, night 18 °C).
Cytological observations and DNA analysis
Chromosome counts were performed on root tips of crosses 1–11, according to the method described by Bervillé et al. (1993). DNA preparation, DNA restriction digestion with EcoRI and HindIII, and Southern blotting were carried out according to the methods described in Lacombe et al. (1999). Progeny from crosses 1–11 and 14 and 15 were characterized using RAPDs (16 primers A5, A12, A15, A16, A19, B19, C2, C4, C12, C14, C15, C16, C19, E3, E9, E15; Operon Technologies, Alameda, CA, USA) according to Sossey-Alaoui et al. (1998, 1999). Each of these primers amplified at least one fragment specific to the perennial species. Progeny from crosses 12 and 13 involving H. mollis and H. orgyalis as male parents, respectively, was characterized by RFLPs (eight probes); cytogenetic characterization is in progress. Statistica (1998) was used for all computations.
Probes used were obtained from Ouvrard et al. (1996) (Sdi-6, Sdi-8, Sdi-9 and Sdi-10) and from Sarda et al. (1997) (15-desaturase and Tip). Stearoyl-ACP-desaturase (
9-desaturase), and Oleoyl-PC-desaturase (12-desaturase) were also used, according to Lacombe et al. (2001).
Phenotype observations
The following traits were recorded: days from planting to flowering (DF); male-sterility or male-fertility due to restoration of PET1 cytoplasmic male sterility; presence of anthocyanin colour on the petiole; head diameter (cm); branched or not branched; absence of some stamens or small stamens; stainability of pollen using the Alexander method (Alexander, 1969); and achene number in self-pollinated heads (bagged) or in open-pollination conditions.
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RESULTS |
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Progeny from crosses of sunflower pollinated by perennial Helianthus
For 293 cultivated sunflower heads pollinated by H. mollis and 42 pollinated by H. orgyalis, the average number of achenes recovered per pollinated head was 0·87 and 0·95, respectively (range 0–26 achenes per head). The number of embryos or achenes resulting from crosses between sunflower and perennial Helianthus species (pollen donor) was therefore much lower than that resulting from crosses between cultivated sunflower and annual species crosses, which are effected with little or no difficulty (Griveau et al., 1992).
Phenotypic analysis
Pollination by H. mollis.
Of the 165 achenes harvested, 91 germinated and 84 grew into well-developed plants. Twenty-nine were found to be similar to the cultivated sunflower female parent for all phenotypic traits observed, and were thus rated ‘0’ for hybrid traits (Tables 1 and 2). However, 46 plants were identical to the H. mollis pollen donor for the following traits: male-fertility, reduced stamens, branching and anthocyanin. Eighteen plants flowered later than the sunflower line, one flowered earlier; 25 were branched instead of producing a single head; 25 were male-fertile instead of male-sterile; and in 14 of the plants stamens were smaller than usual.
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Pollination by H. orgyalis.
Only 15 embryos were rescued from among the 26 crosses. Thirteen plants were obtained, with nine displaying some dominant H. orgyalis traits (Table 2). One plant flowered later than sunflower and one earlier; one was branched instead of producing a single head; eight were male-fertile instead of male-sterile; and one was male-fertile but with reduced pollen stainability (< 76 %) and poor self-fertility (< 22 achenes per head). Phenotypes of hybrids between PET1 cytoplasm female sunflower and H. orgyalis were therefore of sunflower-type or intermediate. Moreover, 12 male-sterile plants had low backcross fertility (< 73 achenes per head), while male-fertile plants displayed smaller stamens than did sunflower.
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Taking both series of experiments together, of a total of 97 plants, 55 (Tables 1 and 2) displayed genetically dominant traits from H. mollis or H. orgyalis: 19 plants were late flowering and two were early flowering; 26 were branched; and 33 were male-fertile or had reduced pollen stainability or low self-fertility.
Chromosome number and DNA analyses
Cytological observation of 20 hybrid plants derived from expts 1–11 indicated a diploid chromosome number (2n = 34). In progeny obtained by embryo rescue following sunflower x H. mollis crosses, all bands (about 50) from the female were usually observed, in addition to 24·6–49·2 bands from the male parent. The minimum (24·6) and the maximum (49·2) values assume that every fragment from the male was heterozygous or homozygous in the H. mollis parent, respectively. ‘Sunflower x H. mollis’ or ‘sunflower x H. orgyalis’ progeny derived by embryo rescue had, on average, 6·5 RFLP fragments. We expected a minimum number of 25·6 and a maximum of 51·2 (Table 3). Only plant R90, derived from H. orgyalis (Table 2), could be considered to have a classical hybrid constitution with 37 out of a possible 57 RFLP fragments.
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In classical experiments involving pollination of sunflower by H. mollis all RAPD bands and RFLP fragments from the female parent were found in the progeny when the cultivated sunflower was the female parent. In contrast, an average of only 1·1 RAPD bands from the male parent were found in these progeny (Tables 1–3). Of the 72 plants obtained from pollination by H. mollis presenting introgressed fragments, RAPD and RFLP marker analyses revealed 41 introgressed plants. Of the 13 plants obtained from pollination by H. orgyalis, eight were introgressed. In 12 hybrid plants, novel RFLP fragments not seen in the parents were detected with Sdi-6 or Sdi-10 probes.
Relationships between phenotypes and fragments
Phenotypically, hybrid plants displayed considerably higher numbers of introgressed or recombined fragments in both interspecific crosses. In expts 1–11, plants obtained from mature achenes showed an average of 1·55 fragments from the male parent compared with 0·46 fragments from the sunflower phenotype. Plants obtained after embryo rescue in crosses 12 and 13 displayed 8·35 fragments diagnostic of hybrid status compared with 0·20 (Table 3). However, the introgressed hybrid plants did not display the expected hybrid profile assuming a combination of fragments from sunflower and the wild parent. Plants derived from mature achenes obtained naturally following pollination of sunflower heads by H. mollis showed 1·55 fragments compared with a minimum of 26·1 (for crosses 1–11), whereas those plants obtained after embryo rescue following similar crosses displayed an average of 8·35 fragments compared with a minimum of 25·2 (crosses 12 and 13) expected from the male parent.
For plants obtained from mature achenes for crosses 1–11 and for plants obtained after embryo rescue in crosses 12 and 13, highly significant correlation coefficients (r = 0·49, P = 0·000; r = 0·67, P = 0·001, respectively) were found between the number of introgressed fragments and the occurrence of hybrid traits (Table 3). A smaller number of introgressed fragments were detected in plants derived from normal achenes rather than by embryo rescue. Minimum expected numbers of introgressed fragments are comparable: 24·6 for crosses 1–11 and 25·5 for crosses 12 and 13 (Table 3). However, when averaged, progeny from crosses 1–11 obtained from normal achenes, yielded 1·13 introgressed RAPD bands, and 1·52 hybrid traits. Progeny from crosses 12 and 13 derived by embryo rescue techniques yielded 6·5 introgressed RFLP fragments and 2·36 hybrid traits (Table 3).
H. mollis pollinated by sunflower.
Helianthus mollis pollinated by sunflower yielded 2·1 viable embryos per pollinated head (n = 403 heads), with a range of 0–32 embryos per head. Hybridization success was therefore low. Some lethal albinos plantlets were produced in vitro; the results obtained from 22 viable hybrid plants are presented (Table 4). Helianthus mollis is branched with sessile leaves whereas sunflower is not branched (single headed) or branched at the top with long petioles. All ‘hybrid’ plants were branched with sessile leaves, and are thus comparable to H. mollis. All plants which had properly developed anthers possessed viable pollen (stainability > 75%), but some antherless male-sterile hybrid plants were produced, probably as a result of the male-sterile status of the H. mollis 286 wild parent. The hybrid plants were either self-pollinated or backcrossed to sunflower. Crossed achene set was on average less than one achene per head, compared with several achenes per head for the H. mollis controls.
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When H. mollis was the female parent, all H. mollis RAPD or RFLP bands were observed in progeny. Between zero and six sunflower bands were found in these progeny, although a range of 26–31 sunflower bands was expected on the basis of RAPD analysis in parental lines (Table 4). Natural crossing between ‘H. mollis x sunflower’ or vice versa was not observed. To obtain progeny, the pollen of one species had to be placed on the stigma of the other species. On average, classical mature achene set was very low (less than one achene per head, for several hundreds of heads pollinated) in both combinations of male and female parent. All plants observed were diploid (2n = 34). The embryo rescue technique increased the recovery of hybrid phenotype plants three-fold.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Results have revealed that progeny derived from interspecific crosses differ due to: (a) the technique used to obtain the progeny; and (b) partial hybridization; this being important in the exploitation of genetic diversity even when crosses are incompatible with foreign material. Moreover, partial hybridization is poorly documented except in rice and Zizania.
In Helianthus, the number of descendants is higher following natural maturation of achenes (62) than following embryo rescue (22), but early embryo rescue is more efficient for recovering hybrid traits in progeny (17/22 compared with 38/62). This is supported by the range of RAPD bands from H. mollis transmitted to the progeny: more bands are detected following embryo rescue (8·35) compared with mature achene set (1·55). On average, embryo rescue enhances the contribution of the H. mollis genome in progeny.
When sunflower was pollinated by H. mollis, progeny tended to resemble sunflower, whereas when H. mollis was pollinated by sunflower, the progeny looked like H. mollis. The absence of symmetry in such crosses is not usual and appeared to be the case in several independent experiments and also with other perennial Helianthus species (not shown).
The RAPDs chosen were specific to the perennial genome (according to Sossey-Alaoui et al., 1998). We assumed that they are homozygous and consequently expected a transmission ratio of 1 : 0 in F1 progeny. This was observed when transmission occurred from the female to the hybrid progeny. However, the male contribution was lower than expected. In both directions and in both interspecific hybrids, plants do not manifest the ratios expected of true F1 hybrids. The same tendencies were observed in all crosses. The phenotypes and genotypes were therefore strongly biased towards the female parent, as shown by the reciprocal crosses (Table 4). In parallel, RAPD bands or RFLP fragments from the female parent were conserved. New non-parental RFLP fragments were detected with two probes in hybrid plants prior to meiosis in crosses 12 and 13. This unusual observation confirms the results of Natali et al. (1998), indicating the alteration of heterochromatin and RFLPs in the first interspecific hybrid generation. Extensive genome changes have been observed in newly synthesized Brassica allopolyploids (Song et al., 1995). Reciprocal allopolyploids were studied and a cytoplasm effect was not excluded. However, preliminary data suggested that chromosome rearrangements in relation with aberrant meioses could be a major factor contributing to genome change. Differences between reciprocal crosses and the appearance of recombined fragments argue against complete fertilization followed by chromosome elimination. In contrast, these facts suggest that the male genome is disturbed and only a part of it is able to mix with the entire female genome. Studies are underway to investigate the events behind chromosome doubling but it is too early to propose an explanatory model.
Similar observations of partial hybridization have been reported by Liu et al. (1999a) for rice lines with introgressed traits from Zizania latifolia. Three introgressed lines obtained using repeated pollination procedures displayed the same rice sequences whereas they differed for Zizania markers. Moreover, Liu et al. (1999b) and Liu and Wendel (2000) have reported changes in DNA methylation patterns and retrotransposon activation, respectively, in such introgressed rice lines. Other strange features have been reported by Wendel et al. (1995) who observed rapid sequence changes followed by homogenization in cotton interspecific hybrids. In wheat, Feldman’s group (Liu et al., 1998a, b) reported changes in low-copy non-coding and coding DNA sequences in newly synthesized amphiploids of Triticum x Aegilops.
If they occur prior to meiosis, genomic rearrangements are possibly very different in nature compared with results of hybridization between barley and H. bulbosum (Kasha and Kao, 1970). In this case, genome rearrangements or a haploidization events are observed as a direct consequence of alien pollination, before meiosis in the hybrids. Early elimination of alien fragments or chromosomes is possible, as observed by Kasha and Kao (1970) in Hordeum. However, Laurie and Reymondie (1991) and Riera-Lizarazu et al. (1996) showed that in similar experiments with wheat and oat pollinated by maize, some partial hybrids were obtained. Recent results from Natali et al. (1998) have also been obtained before meiosis on Helianthus hybrid plants between cultivated sunflower as female and H. tuberosus as the male parent and were interpreted as a consequence of genomic shock (McClintock, 1984). They showed that F1 plants differed in DNA content due to variation in moderately repeated sequences. These plants are currently being studied to detect eventual changes in repeated sequence families.
In conclusion, we stress the implication of partial hybridization. It is easy to overlook this phenomenon in other crops crossed with their wild relatives because most of the plants do not display the expected hybrid pattern but instead resemble the female line leading one to conclude that the cross has failed. We detected this phenomenon because we used diagnostic markers from both parents and because we systematically checked all progeny. Partial hybridization has implications for breeding purposes and may allow the exploitation of genetic diversity of more distant species than first expected, when interspecific crosses have apparently failed. However, differences in crosses between cultivated and wild species and the corresponding reciprocal crosses may alert the breeder to the occurrence of partial hybridization. There may also be important consequences for evolution and speciation since apparently incompatible species may exchange genes once speciation has separated them. This could lead to phylogenetic relationships being reconsidered, not only in Helianthus but also in other genera.
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ACKNOWLEDGEMENTS |
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We acknowledge the helpful assistance of Alain Gil and Pierre Lacombe.
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