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Crop Science 40:256-263 (2000)
© 2000 Crop Science Society of America

PLANT GENETIC RESOURCES

Geographical Distribution of a Chromosome 7C and 17 Intergenomic Translocation in Cultivated Oat

E.N. Jellena and J. Bearda

a Brigham Young University, Department of Agronomy and Horticulture, 275 WIDB, Provo, UT 84602 USA

enj{at}email.byu.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 
Previous cytogenetic and molecular genetic investigations have shown variation for the presence of intergenomic translocation segments on chromosomes 7C and 17 of common cultivated oat (Avena sativa L., 2n = 6x = 42) and red oat (A. byzantina K. Koch, 2n = 6x = 42). The objective of this work was to determine the geographic distribution of these translocation segments in 197 landraces and cultivars using C-banding. Genotypes were selected primarily on the basis of diversity of geographic origins, particularly within the Mediterranean-Near Eastern center. Eighty-nine percent of traditional A. byzantina-type accessions, mostly from the lowland Mediterranean basin and Indian subcontinent, were of the nontranslocation type. Ninety-seven percent of traditional A. sativa and hulless (A. sativa subsp. nuda) genotypes possessed the 7C-17 translocation ({Delta}7C-17) segments. Presence or absence of the {Delta}7C-17 was more loosely associated with spring vs. winter growth habit, but still highly significant. The results support the hypothesis that common cultivated oat and red oat are distinct races of the hexaploid biological species and were domesticated independently of one another.

Abbreviations: {Delta}7C-17, 7C-17 translocation • Dp-Df, duplicate-deficient • NSGC, USDA National Small Grains Collection


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results
 Discussion
 REFERENCES
 
THE FOUR COMMONLY RECOGNIZED hexaploid oat taxa—A. byzantina, A. fatua, A. sativa, and A. sterilis (all 2n = 6x = 42)—represent a single, interfertile "biological" species (Ladizinsky, 1988). Vavilov (1926) and Malzew (1930) determined that the cultivated A. byzantina and A. sativa originated as secondary crops and were disseminated outward from an Anatolian center of origin as weeds of emmer [Triticum turgidum subsp. dicoccon (Schrank) Thell.]. Common oat (A. sativa) and red oat (A. byzantina) have been extensively interbred in modern cultivar development, although in older varieties the two taxa are differentiated primarily by their heterofracturing vs. basifracturing rachilla, respectively (Stanton, 1961; Leggett, 1992). A cytogenetic basis for differentiating the two subspecies of cultivated hexaploid oat first arose when a submetacentric form of chromosome 7C and an intergenomic translocation segment on 17 were observed, always together, in 31 of 32 spring oat cultivars (A. sativa-type) of northern European origin (Jellen et al., 1996). Conversely, in 18 of 23 fall-sown red oat (A. byzantina-type) genotypes, chromosome 7C was metacentric and the intergenomic translocation segment on 17 was absent. The A. sativa-type forms of chromosomes 7C and 17 were found to predominate in the majority of sampled A. sterilis populations from throughout the Mediterranean and Near East (Zhou et al. 1999). However, several localized populations in North Africa and the Levant had the A. byzantina forms of these chromosomes. A concentration of A. sterilis populations from northern Mesopotamia, northwest Iran, and southeastern Turkey was also found to be lacking the translocation forms of these chromosomes.

Oat RFLP Linkage Groups 3 and 24 from the `Kanota' (A. byzantina) x `Ogle' (A. sativa) map (O'Donoughue et al., 1995) represented portions of chromosomes 7C and 17, respectively, as determined by aneuploid mapping analysis (Kianian et al., 1997). Moreover, a putative translocation breakpoint on hexaploid oat Linkage Group 3 was identified by mapping RFLP markers to chromosome 17 monosomics in Kanota, which is lacking the {Delta}7C-17 (S. Fox, 1999, personal communication). The potential agronomic importance of these two chromosomes was highlighted by RFLP-based QTL mapping studies, which indicated that regions on one or both of these chromosomes contain important genes affecting plant height, maturity, and physiological responses to vernalization (Siripoonwiwat et al., 1996; Holland et al., 1997).

Although reciprocal translocations are found in wild (McMullen et al., 1982) and cultivated (Ladizinsky, 1970; Singh and Kolb, 1991; Wilson and McMullen, 1997) hexaploid oat genotypes, their evolutionary significance in Avena is unknown. However, Rajhathy and Thomas (1974) reasoned that translocations were probably a major genomic divergence mechanism in allopolyploid oat evolution. Translocations may have profound effects on recombination through physical suppression of pairing around the breakpoint (Burnham, 1934; Bridges and Brehme, 1944), as well as inviability of duplicate-deficient (Dp-Df) spores arising from interstitial recombination in translocation heterozygotes (Sansome, 1932).

Stebbins (1971) reasoned that translocations might be naturally selected for and fixed in populations when they placed genes conferring a collective adaptive advantage into a common linkage group (adaptive gene cluster hypothesis). We wondered if such a phenomenon might also be operative under artificial selective pressures in cultivated environments. If so, one manifestation might be the correlation of a specific translocation with a particular cultivated growth habit, such as the spring vs. winter habit in oat.

The main objective of our study was to determine the geographic distribution of chromosomes 7C and 17 in landrace oat genotypes from Asia, southern Europe, and North Africa. A correlation of the {Delta}7C-17 with spring-habit genotypes, or conversely of normal 7C and 17 with winter-habit genotypes, would suggest the presence of important adaptive gene(s) near one or both translocation breakpoints. Additionally, a close association between translocation and heterofracturing, or between nontranslocation and basifracturing, would provide support for the hypothesis that A. sativa and A. byzantina represent distinct races of cultivated oat that arose independently from weedy progenitors, as molecular phylogenetic data indicate (Zhou et al., 1999).


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results
 Discussion
 REFERENCES
 
A total of 140 accessions from the USDA National Small Grains Collection (NSGC) (Table 1) were subjected to C-banding using the protocol of Jellen et al. (1993a), except that Giemsa stain (Sigma, St. Louis, MO) was used instead of Wright's stain. This set of accessions included 73 A. byzantina, 49 A. sativa, seven A. sativa subsp. nuda, and 11 that were heterogeneous for basal rachilla abscission mode. Accessions were selected for study on the basis of diversity of geographic origin, focusing on the Mediterranean-Near Eastern center of origin of Avena (Baum, 1977). Landraces were also specifically selected from the NSGC to represent the range of geographic regions in Turkey, the country that had previously been shown to harbor the greatest amount of genetic diversity for the putative wild hexaploid progenitor A. sterilis (Phillips et al., 1993; Zhou et al., 1999).


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  		Table 1 Oat accessions examined via C-banding, including abscission type or race, growth habit, and presence or absence of the 7C-17 translocation. Data on oat varieties examined previously in Jellen et al. (1996) are also presented

 
In addition, we examined the association between 7C-17 chromosome forms and abscission type more closely in three Turkish landrace accessions heterogeneous for abscission type: PI 168078 (Manisa), PI 166969 (Hatay), and PI 167378 (Icel). In each accession, seed from two heterofracturing and two basifracturing spikelets were germinated and their root-tip chromosomes C-banded. Two Moroccan oat cultivars, INRA 8202 and Tissir, were also C-banded. Observations on 32 A. sativa and 23 A. byzantina cultivars that were previously C-banded (Jellen et al., 1996) were also included.

Chi-square values, using Yates' continuity correction with one degree of freedom, were used to test significance of the associations between presence or absence of the translocation with either rachilla fracturing mode or growth habit. Cytological preparations were examined on a Zeiss Axioskop microscope (Carl Zeiss, Inc., Thornwood, NY) and captured using a SenSys videocamera system (Photometrics, Tucson, AZ). At least three seedlings per accession were examined. Accession information was GIS-mapped using the Xerox PARC Map Viewer available on the World Wide Web (http://pubweb.parc.xerox.com), using latitude and longitude collection information from the NPGS/NSGC web site (http://www.ars-grin.gov/npgs/).


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
Discussion
 REFERENCES
 
Translocation scores for lines examined in this study and previously in Jellen et al. (1996) are presented in Table 1. These data were geographically presented in Fig. 1 for a subset of landrace oat accessions from the eastern Mediterranean region. Photographs of C-banded chromosomes of representative accessions without (Fig. 2) and with (Fig. 3) the {Delta}7C-17 chromosomes are also presented.



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  		Fig. 1 Geographic distribution of a subset of oat landraces from the eastern Mediterranean region

 


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  		Fig. 2 C-banded somatic chromosomes of oat accession CIav 5248, a landrace from Canakkale Province, Turkey, which is lacking the 7C and 17 translocation forms. Magnification is 1000x

 


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  		Fig. 3 C-banded somatic chromosomes of oat accession PI 287308, a landrace from Iran, showing 7C-17 translocation. Translocated segments are identified by brackets. Centromeres are identified by asterisks. Magnification is 1000x

 
In general, oat landrace accessions from lowland Mediterranean coastal climates were lacking {Delta}7C-17. The main exceptions to this were a group of four landraces from the Nile Delta in Egypt and five of 10 accessions from Morocco. In addition, nine of 11 accessions from the Indian Subcontinent (India, Nepal, and Pakistan) were lacking the translocation.

In contrast, {Delta}7C-17 appeared to predominate in interior Turkey, among hulless oat accessions from the Far East, in the Ethiopian Highlands, and among old oat cultivars of northern Europe. Within the coldest northeastern quadrant of Turkey, north of 39°N and east of the 35°E, all 14 accessions possessed {Delta}7C-17. In addition, eight of nine accessions from China or Mongolia had the translocation forms of 7C and 17, the exception being a hulless breeding line from the Grassland Research Institute of Inner Mongolia, China. All four accessions from the Ethiopian Highlands of East Africa also had {Delta}7C-17.

There was a strong association between abscission type or race and {Delta}7C-17 type, with a C2 value of 134.7 (P <= 0.001) (Table 2) . Of the accessions homogeneous for the basifracturing A. byzantina type of rachilla, 81 (89%) were uniform for the nontranslocation forms of 7C and 17 and only five (5%) were homogeneously {Delta}7C-17. For the accessions uniform for the A. sativa type of heterofracturing rachilla, 77 (97%) had {Delta}7C-17. Of the ten hulless accessions (previously classified as A. nuda), nine were of the {Delta}7C-17 type, the exception being the breeding line (PI 447272) from Inner Mongolia mentioned above. After separating heterofracturing and basifracturing spikelets of two heterogeneous Turkish landrace accessions, PI 168078 (Manisa) and PI 167378 (Icel), seed from the heterofracturing spikelets gave rise to seedlings that were {Delta}7C-17, and the basifracturing seed produced plants with normal 7C-17. With PI 166969 (Hatay), one of the four heterofracturing seed examined produced a {Delta}7C-17 plant while the other, and both basifracturing, seed produced plants that were normal 7C-17.


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  		Table 2 Numbers of oat accessions or cultivars of each abscission type or race homogeneous or heterogeneous for the normal (7C,17) and translocation ({Delta}7C-17) types of Chromosomes 7C and 17

 
The association between winter vs. spring growth habit and {Delta}7C-17 type was also highly significant, with a C2 value of 41.7 (P <= 0.001) (Table 3) . Of the 87 accessions uniformly classified in the NSGC as winter-habit in this study, 60 (69%) were found to be homogeneous for the nontranslocation chromosome forms. Of the 99 accessions uniformly classified as spring-habit, 72 (73%) had the {Delta}7C-17 karyotype. Many of the spring-habit, nontranslocation and winter-habit, translocation accessions were from transition-zone areas such as Turkey (14) and the Balkans (5). The hulless oat accessions, which were primarily {Delta}7C-17, had both spring and winter growth habits. Additionally, seven of the eight Indian uniform or mixed A. byzantina accessions had spring-habit, nontranslocation types.


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  		Table 3 Numbers of oat accessions or cultivars of each growth habit type homogeneous or heterogeneous for the normal (7C,17) and translocation ({Delta}7C-17) types of chromosomes 7C and 17

 

   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
REFERENCES
 
Cytogenetic expectations regarding recombination in a translocation heterozygote are presented in Fig. 4 . Theoretically, if a gene(s) for abscission type or a spring-adaptive gene cluster or QTL were located on the translocated segment of chromosome 7C or 17 (Fig. 4a), the likelihood of recombination in translocation-gene heterozygotes would be a function of two factors: (i) the physical distance between the gene(s) and the breakpoint and (ii) the magnitude of chiasma disruption due to mispairing in the vicinity of the cross. Alternate disjunction would give rise to {approx}66% functional gametes and adjacent-1 disjunction would produce {approx}33% Dp-Df gametes, assuming equal frequencies of alternate and adjacent disjunction and abortion of grossly deficient spores arising from adjacent-2 disjunction.



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  		Fig. 4 Cytological consequences of single-crossover recombination between a breakpoint and a gene in (a) the translocation segment, (b) the interstitial region, and (c) in both interstitial regions in a translocation heterozygote

 
However, if the gene(s) in question were located on the interstitial segment, a single recombination event between the gene(s) and breakpoint, in one of the two interstitial segments, would result in normal (non Dp-Df) gametes only under adjacent-1 disjunction (Fig. 4b). Alternate disjunction, again assuming random chromatid segregation and nonrecovery of adjacent-2 gametes, would yield {approx}66% potentially viable Dp-Df gametes. Moreover, a crossover event proximal to the gene(s) in question on the interstitial segment would produce the same result, without producing a "recombinant" type for abscission- or habit-translocation. The occurrence of simultaneous, single crossovers in both interstitial segments would result in 66% non–Dp-Df gametes, again through alternate disjunction (Fig. 4c).

The commonality of translocations in oat cultivars (Singh and Kolb, 1991), the relative stability of certain Dp-Df oat genotypes, and the relatively frequent appearance of fatuoids and other aberrant types in oat cultivars indicate that duplication-deficiency may be widespread in this crop (Wilson and McMullen, 1997). However, Dp-Df genotypes for the segments of chromosomes 7C and 17 in question would be easy to detect using C-banding because of the distinct morphology of these chromosomes. Given the absence of chromosome 7C-17 Dp-Df genotypes among the seven potential "recombinant" types for abscission (Table 2) and at least 44 for growth habit (Table 3), the genes controlling these characters are probably tightly linked to the breakpoint(s) if they are on interstitial segments.

The close association between abscission type or race and 7C-17 chromosome forms supports the hypothesis put forward by Zhou et al. (1999) that A. byzantina and A. sativa represent two distinct races of cultivated oat, each having a separate origin from the wild, weedy hexaploid ancestor A. sterilis. Avena byzantina cultivars displayed the greatest degree of genetic similarity with a cluster of A. sterilis accessions predominantly from northern Mesopotamia. On the other hand, A. sativa cultivars were most similar genetically to a cluster of A. sterilis genotypes from eastern Anatolia (Zhou et al., 1999). Whether one or both of these cultivated forms arose via a weedy floret-dispersed oat species intermediate is uncertain; however, a large proportion of A. fatua, A. hybrida, and A. occidentalis accessions that we are studying also possess {Delta}7C-17 (unpublished results). The high percentages of A. sativa (97%) and A. byzantina (89%) accessions with and without {Delta}7C-17, respectively, indicate that if the gene(s) controlling rachilla fracture is on one of the two translocated segments, it is probably closely linked to the breakpoint.

Our results support previous QTL mapping studies indicating that important adaptive gene(s) related to spring habit are located on chromosomes 7C and 17 (Siripoonwiwat et al., 1996; Holland et al., 1997). In general, spring-habit oat landraces are restricted to regions with relatively moist, mild summers or winters that do not experience prolonged subfreezing temperatures. In contrast, winter-habit landraces are found in regions with relatively most, mild winters and hot summers, as around the Mediterranean Sea. While oat is predominantly self-pollinating, gene exchange through occasional outcrossing between overlapping spring-habit A. sativa and winter-habit A. byzantina landraces in transitional zones could account for {Delta}7C-17 winter types and nontranslocation spring types. Although winter races would be expected to reach anthesis earlier than spring types in a transitional zone, odd years without a sufficient vernalizing cold period, for example, might result in concurrent maturation of winter and spring races. We would also expect a certain amount of genetic exchange to occur between Mediterranean-region A. byzantina landraces and cohabiting A. sterilis or A. fatua weeds having {Delta}7C-17 (Zhou et al., 1999). Following the arguments above regarding position of the gene(s) controlling spring habit, if the gene(s) in question is on a 7C-17 translocated segment, it is most likely in a more distal position than the gene(s) for rachilla fracture.

The historical existence of winter-habit A. sativa and A. byzantina landraces in places such as Britain complicates the interpretation of these results (Coffman, 1961). The international movement of oat by ancient nomadic peoples, European colonialists, and modern plant breeders has also obscured our understanding of the origin of cultivated, hexaploid Avena. Nevertheless, due to the predominance of {Delta}7C-17 types in the Far East and the predominance of normal 7C-17 in Indian Subcontinent oat landraces, it is more likely that oat arrived in China via Central Asia than over the Himalayas. Coffman (1961) had previously traced the origin of oat in Chinese literature back to approximately 500 A.D.

Earlier work with oat aneuploids indicated there may also be a critical gene or block of genes for viability within the translocated portion of chromosome 7CL. Although a near-complete aneuploid series is now available in the Sun II (A. sativa, {Delta}7C-17) genetic background, including fully fertile {Delta}7C nullisomics, one of three chromosomes not represented by an aneuploid is {Delta}17 (Jellen et al., 1997). Vigorous {Delta}7C nullisomics were extracted from Sun II, originally as lines VII and XIV (Hacker and Riley, 1965). Phenotypically, these nullisomics closely resemble the disomic Sun II parent. Conversely, although vigorous chromosome 17 monosomics are available in the Kanota genetic background, we have yet to isolate chromosome 7C-deficient aneuploids in the Kanota background (Jellen et al., 1993b). In addition, efforts to isolate viable, stable Dp-Df lines from among the progeny of A. sativa x A. byzantina crosses in our laboratory have been unsuccessful, although we are currently cytogenetically characterizing Kanota x Ogle recombinant inbred lines reported to be Dp-Df for groups of chromosome 7C-17 RFLP markers. Of course, if an essential viability gene(s) is located within the translocated portion of chromosome 7CL, this could account for the lack of Dp-Df genotypes in the set of 197 examined in this study.

Future research should focus on mapping rachilla-fracture and spring-habit genes, on identifying other genes and markers on or near the 7C and 17 translocation segments, and on studying the effects of the two breakpoints on recombination in translocation heterozygotes. In addition, studies designed to examine the evolutionary position of A. fatua among the hexaploid oat species should be pursued.


   ACKNOWLEDGMENTS
 
The authors are grateful for the assistance of H. Bockelman of the National Small Grains Collection, USDA, in providing germplasm for this study. This research was supported by Brigham Young University. In addition, J. Beard was supported by an undergraduate scholarship from the BYU Office of Research and Creative Activities (ORCA).

Received for publication January 7, 1999.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 




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