the role of the lodicule and epiblast in determining natural hybrids of avena sativa ...

The role of the lodicule and epiblast in determining natural hybrids of Avena sativa x fat~ra in cultivated oats1

BERNARD R. BAUM Plant Research Institute, Canada Department of Agriculture, Ottawa, Canada

Received September 9, 1968

BAUM, B. R. 1969. The role of the lodicule and epiblast in determining natural hybrids of Avena sativn X jatua in cultivated oats. Can. J. Botany, 47: 85-91.

Most reports on natural hybrids between A. sativa and A. f o t ~ ~ a are not supported by definitive ex- perimental evidence. Those authorities who have proved that natural hybrids do occur were assisted by the fact that both parents were known and occurred with the putative hybrids. The present study was conducted on samples of foundation plots from Canadian origin. Very useful characters were found in the lodicules and in the epiblasts which made it possible to detect F1 hybrids with precision in seeds of cultivated oats irrespective of their external morphologic markers. A hypothesis that the epiblast of the Fatua-type is dominant over the Sativa-type in F1 hybrids is set forth on the basis of the present findings. Higher frequencies of the Fatua-type epiblast were found in the samples where the lowest frequency of the Fatua-type lodicule was observed, because these samples were cultivars which had in their breeding history more crosses with A. byzantina than the others had. The importance of the lodicule and epiblast characters as markers is reassessed, and their reliability was reevaluated in view of the similarity of characters found in the epiblast of A. fatua and A. byzantina, and in view of the present findings. The taxonomic status of A. fatua and A. sativa is also discussed.

Introduction Some treatments dealing with A. sativa X

Natural hybrids between Avena sativa L. and A. fatua L. have been reported by many authorities, such as Koernicke and Werner (1885), Ascherson and Graebner (1899), Fischer (1900), Zade (1912a, 1912b, 1918), Rouy (1913), Tschermak (1914), CrCpin (1921, 1925, 1928), Schribaux (1925), Thellung (1929), Coffman and Wiebe (1930), Malzew (1930), Derick (1933), Aamodt et al. (1934), Taborda de Morais (1937), and Thurston (1957). Most of other reports of natural hybrids between these two species are contained in local European floras; however, no specific generation data of the hybrids are given. Hybridity was assumed because many intermediate forms between A. sativa and A. fatua had been encountered, recorded, and described. Nevertheless, the descriptions of these intermediate forms, or hybrids, were ill- defined in many treatments, and in some cases very controversial. For instance, Ascherson and Graebner (1899, p. 242) considered A. byzantina Koch to be intermediate form or hybrid A. sativa X jatua; Rouy (1913, p. 124, footnote) came to the same conclusion; Thellung (1929, p. 420) kept A. byzantirza as a subspecies of A. sterilis L., not a hybrid, whereas Taborda de Morais (1937) regarded A. byza~ztina as a distinct species of equal value to A. sativa and A. fatua.

lcontribution No. 698 of the Plant Research Institute.

fatua hybrids approached-the problem either indirectly by producing artificial hybrids which are similar to the suspected ones, e.g. Taborda de Morais, Aamodt et al,, Tschermak; or directly by finding markers, mainly dominant alleles of one of the parents, e.g. CrCpin, Derick, Coffman, and Wiebe. In these studies the parents were definitely known and a marker such as lemma color, was known to be specific to one of the parents. However, such information was not sufficient for the detection of natural hybrids nor to point to the hybridity of a specific individual when the parents were virtually unknown.

The study of natural hybridization between A. sativa and A. fatua has been complicated because of the existence of fatuoids and other different off-types. Many experiments and much research were carried out to explain the origin of fatuoids and off-types. Artificial crossings were made to check the hypothesis that fatuoids originate from natural crossing and to investi- gate the genotype of A. sativa and of A. fatua. These experiments revealed that although A. sativa is predominantly self-pollinating, outcrossing does occur and spontaneous hybrids between varieties and with A. fatua may exist. Many artificial crossing experiments between A. sativa and A. fatua and studies on natural crosses between cultivars are summarized in the review

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86 CANADIAN JOURNAL OF BOTANY. VOL. 47. 1969

TABLE I

Sample C S G A No. 1 C S G A No. 2 C S G A No. 3 Breeders' seeds

Lodicule and Number Number Number Number epiblast of seeds of seeds of seeds of seeds

types scored % scored % scored % scored %

LF EF 38 7.6 34 6.8 5 5.0 1 1 .O

Only EF* 16 3.2 1 0.2 0 0.0 0 0.0

LF Es 2 0.4 5 1 .O 0 0.0 1 1 .O

Ls EF 80 16.0 83 16.6 47 47.0 39 39.0

LS Es 364 72.8 377 75.4 48 48.0 59 59.0 Total 500 100.0 500 100.0 100 100 100 100.0

*Only caryopsis alone present, lodicules absent as a result of threshing.

papers of Coffman and MacKey (1959) and Jensen (1961).

The genetic data derived from these experiments can be summed up as follows: many characters in A. sativa are dominant or partially dominant over A. fatua and many segregate in monomeric ratios in FZ generations. Examples

, of such characters are spikelet separation, floret disarticulation, and awns. Many of these characters are genetically linked, e.g. the "cultivated base", which is a complex of non-disarticulation and non-hairiness together with awnlessness. The latter character complex is dominant in F1 hybrids A. sativa X fatua. It is always easy to recognize the F1 hybrids when the parents are known. Nevertheless, when the parents are unknown, or when cases of partial dominance of one or more characters is present because of the biotype of the parent, it is difficult to recognize an F1 hybrid. There is great variation in some of these characters such as the awn character, which may vary from a completely awnless to a fully expressed awned state in A. sativa. The F1 hybrid is not recognizable either phenotypically or morphologically if it originates from an awned biotype of A. sativa and an unknown A. fatua. Thus many of these inter- njediate forms found and described by Hauss-

knecht (1885, 1892), Zade, Thellung, etc., might not necessarily be of hybrid nature, but different biotypes of A. sativa with respect to the awn.

I have found markers enabling detection of putative F1 hybrids in any kind of material of oats (Baum 1968) and I pointed to the existence of natural hybrids between A. sativa and A. fatun in Canada. These markers are based on some morphological features of the lodicules and epiblasts. In this paper I intend to present additional evidence of natural hybridization between A. sativa and A. fatua occurring in oat fields in Canada. Additional evidence about the reliability of the lodicule and of the epiblast as m,Arkers is presented and their use in the taxonomy of A. sativa and A. fntua are assessed.

Material and Methods Seed material of cultivated oats was obtained from

three different foundation plots. Foundation stock is scrutinously selected by growers for production of seeds true to variety. A sample of each foundation plot is regularly forwarded by the grower to the Canadian Seed Growers Association for Post Control Varietal Verifica- tion Test. Two ounces from each sample was obtained from the CSGA at Ottawa. The first variety "Harmon oats" (CSGA No. 1) was grown in Alberta; 500 seeds were studied. The same amount of seeds was observed in the second sample "Garry oats" (GSCA No. 2) also

FIG. 1. Fatua-type lodicule, X 90. The apex of the lodicule was not taken; it is similar to that in Fig. 3, which is also cut. Note the beginning of the trough at the base of the lodicule. No side lobe is present; Com- pare with the other type in Figs. 8, 9, 10, 11, 13.

FIG. 2. Fatua-type epiblast in A. fattin, X 90. FIG. 3. Sat~va-type lod~cule, X 69. Note the beginning of trough at the base, and the obliquely attached ,

side lobe at the back of the cylindrical base of the main body of the lodicule., FIG. 4. Sativa-type epiblast, X 90. FIGS. 5, 6. Fatua-type epiblasts found in introgressants of A. sativa-byzantina, X 90. FIG. 7. Detail of the base of a Fatua-type lodicule, X 90. Note the thin wing-like structure obliquely at- . -

tached at the base of the lodicule. .,

FIG. 8. See caption for Figs. 9-11.

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BAUM: AVENA SATIVA AND AVENA FATUA 87

grown in Alberta. In the third sample, (CSGA No. 3) typical Fatua-type epiblast (EF). In Figs. 3 and 4 "Russell oats" grown in Ontario, only 100 seeds were are shown a typical ti^^-^^^^ lodicule (L,) checked. The fourth sample, also "Russell oats" consisted of the original Breeder's Seeds obtained from the and epiblast Figures and an Ottawa Research Station, CDA, where this variety had EF found in A. sativn (see discussion). Figure 7 been originally produced (Zillinsky 1961). The four shows the wing-like structure of the LF. Figures samples consisted of entire florets, with the exception of 8. 9, 10, 11, and 13 demonstrate some variable an occasional caryopsis.

The seeds were soaked in simmering water and dis- sizes and shapes of the side lobe of the Ls. sected. Lodicules and epiblasts were processed in the same Figures l2, l4, 15, and l6 the care manner as was described by Baum (1968). Examinations that must be exercised in the examination and were done and photographs were taken with the aid of interpretation of the TYPE of the lodicule when a Carl Zeiss microscope with Nomarski interference the side lobe is folded or small and folded. contrast equipment.

Results Discussion The external morphology of the seeds in each

sample appeared uniform. All four samples consisted of oats true to variety, i.e. lemma awnless, and glabrous; floret with a "cultivated base7' complex. For every sample only an in- significant number of wild oats and off-types per twenty thousand plants was reported but these had been previously discarded from each sample by the grower. In spite of the external uniformity of the seeds, I was able to separate each sample into several groups as show11 in Table I, using lodicule and epiblast characters.

According to my previous findings (Baum 1968), F1 hybrids between A. sativa and A..fntua might be detected with the aid of lodicules. Such an F1 hybrid has a complete similarity to the A. sativn parent, but has a Fatua-type lodicule. Thus, the frequency of hybrids in each sample can be computed by adding all LF's (Fatua-type lodicule) in each sample. CSGA No. 1 has 7.6 + 0.4 = 870 F1 hybrids; CSGA No. 2 has 6.8 + 1.0 = 7.8% F1 hybrids; CSGA No. 3 has 575 F1 hybrids; and in the Breeder's Seeds only 2% F1 hybrids were scored.

The epiblast is variable as can be seen in Table I, and this confirms previous findings (Baum 1968).

Detailed descriptions accompanied by sche- inatic drawings of the Sativa-type lodicule and of the Fatua-type lodicule were given previously. Photomicrographs of the lodicules and epiblasts as they appear under an interference contrast equipped microscope are shown. Figures 1 and 2 show a typical Fatua-type lodicule (LF) and a

Harrington (1932) reported that cross-pollina- tion in oats occurs up to 9.82y0 in the Liberty hull-less variety. Coffman and Wiebe (1930) reported that natural crossing between black aberrants of Richland oats and the normal Richland oats to be as high as 6.62y0 and that in addition the progeny contained O.82yO yellow- kernelled aberrant and a small percentage of fatuoids. Similar results were also summarized in the review paper by Jensen (1961). On the basis of observed monogenic characters in naturalized populations of A. jatua in California, Imam and Allard (1965) estimated the outcrossing in these populations to range from 1 to 12%.

Our results fall virtually within the range of of those previous findings, 8.0, 7.8, 5.0, and 2.075 in CSGA No. 1, CSGA No. 2, CSGA No. 3, and in the Breeder's Seeds, respectively. This demonstrates the difficulties in maintaining the purity of oat varieties. The sample of the Breed- er's Seeds (Russell oats) examined was taken from a later generation of the original stock which had been maintained at a high level purity. Although the purity had been carefully con- trolled, the stock of the Breeder's Seed was contaminated by A. .fatun. Furthermore, the same variety, Russell oats, which I obtained from the foundation plots was more contaminated by A..fntua than the Breeder's Seeds. The two other samples were grown in Alberta where there is probably a higher chance for crossing with A. .fntua than in Ontario, where the two samples of Russell oats were grown, and there-

FIGS. 9-11, 13. Sativa-type lodicules with a very short side lobe, X 90. Note the lobe with its typical basal structure which can never be confused with the wing-like structure of the Fatua-type lodicule (Fig. 7).

FIG. 12. A Sativa-type folded lodicule with a small side lobe, X 90. Note the side lobe on the right side. FIGS. 1416. Sativa-type lodicules each with a folded side lobe, X 90.

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88 CANADIAN JOURNAL OF BOTANY. VOL. 47. 1969

fore the higher percentage of F1 hybrids in the two Alberta samples. Nevertheless, this might not be the only explanation for higher percentage of hybrids in the Alberta samples since the rate of outcrossing depends on the biotype and on various ecologic factors such as temperature, wind, and humidity.

The lodicule is a reliable tool not only for distinction between A. fatua and A. sativa, but also for detection of putative F1 hybrids, and particularly for finding those hybrids among the cultivated oats in view of the fact that both the cultivated oats and the hybrids appear identical in their external morphology. This is, I think, of vital importance for the plant breeder and geneticist when considering purity of material.

The epiblast is an organic part of the embryo of the seed. As such it belongs always to the next generation. Hence, its morphology might vary according to the origin of the pollen. Thus, if an A. sativa plant was pollinated by A. fatua the morphology of the epiblast might be of a different expression, but in the same floret, the lodicule and other floret parts would not be affected. In the latter case, A. sativa being the mother plant, the lodicules will remain of the Sativa-type. In self-pollinated flowers, or in flowers which have been pollinated by the same species, we should expect that the lodicules and epiblasts will constantly remain of the same type, i.e. A. sativa will have Sativa-type lodicules and Sativa-type epiblasts, A. fntua will have Fatua-type lodicules and epiblasts. This is in accordance with previous findings as well as with present ones (LsEs of Table I).

I was not able to get material of seeds of F1 crosses to check directly whether the Sativa- type of epiblast is dominant over the Fatua-type or vice versa. Because of this, I will attempt to fit the observed frequencies to those theoretically expected.

Let us first assume that the Sativa-type epiblast (Es) is dominant over the Fatua-type (EF), and examine the reciprocal crosses.

A. sativa X A. fatua 9 : Ls Es or LsEF 9 : LFEF

Seeds: LsEs or LsEF Seeds : LEEs or LEEF F1 : A. sativa phenotype and LF 3Es : lEF expected when EF recessive

Avena sativa has the typical external morphology of the cultivated oat and a Sativa-type

lodicule (Ls), but in many cases, depending on the biotype, it has variable amounts of the two epiblast types (Es and EF). Hence, these markers appear in A. sativa in the following combinations L,Es and LsEF, whereas A. fatua has only LFEF.

When an A. sativa plant has been cross- fertilized by an A. fatua male plant, the typical external morphology of the A. sativa plant remains the same, including the Ls lodicule, because cross-fertilization does not affect the mother plant. The other marker which might have been affected is the epiblast since it rep- resents the next generation and may be of hybrid nature. However, we assumed that ES is dominant so that this phenotype has not changed. Nevertheless, the F1 hybrid plants if grown from these seeds will have LF lodicules and the external morphology of A. sativa according to my previous findings. These F1 hybrids will produce gametes with Es and EF genes, respectively. Some plants might be selfed, others cross- pollinated, and in any event there is an equal chance of mating. Consequently, at least 50% of F1 hybrids are expected to have Sativa-type phenotypically expressed epiblasts. The present findings do not show such a tendency (Table I), nor do the previous findings because only the LEEF combination was found in all the F1 putative hybrids which I detected.

When A. fatua, in turn, is the mother plant, then we expect to find A. fatua with LEES seeds. This has never been found either in the previous survey in the Canadian material ( B ~ L I ~ 1968) or in the present survey (Table I), as well as other unreported tested samples.

Let us assume, now, that the Fatua-type epiblast (EF) is dominant over the Sativa-type epiblast (Es).

A. sativa X A. fatun 9 : LsEs or L s E ~ 9 : LFEF

Seeds: LsEF Seeds: LEEF F1 : phenotype of F1 : phenotype of A. sativa and LF A, sativa and LF

3EF : lEs expected when Fatua-type is dominant

As shown, the seeds of cross-pollinated A. sativn by A. fatua are LsEF plants, and these seeds will produce F1 hybrid plants with LF. Since two different gametes with Es and EE genes, respectively, have the same chance of mating, we should expect to find many F1 hybrids with the phenotype of A. sativa partic-

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ularly with LEEF. This is in accord with the findings. When A. fatua is the mother plant as in the reciprocal cross, the EF of hybrid origin will not be recognized among the ordinary A. fatua seeds because it is the same as the dominant one. In cultivated fields those plants are usually eliminated by man because of their A. fatua external morphology. Nevertheless, if they are not eliminated, for instance in waste fields, the same expectation of mating combination will be expected as well as their phenotypic expression. This will give LEEP in most cases, and LFEs in a few cases. This is in accord with the present findings and also with the previous ones. Consequently, I assume that the Fatua-type epiblast is dominant and the epiblast character is monogenic and not polygenic since all cases which I was able to observe are clear-cut. Detailed genetic work is required to test the significance of this hypothesis.

In my former findings (Baum 1968) I stated that the TYPE of the epiblast varies according to the biotype of A. sativa. This is sl~own in Table I, for it can be noticed that in many cases a Sativa-type lodicule is associated with a Fatua- type epiblast. Percentagewise some cases of LsEF might be interpreted as an A. sativa mother floret that was cross-fertilized by A. fatua, in other words a future F1 hybrid A. sativa X fatua. Nevertheless, not all of these results can be interpreted in this manner, but only a few of them, because the rate of outcrossing was never found to be as high as that reported in published records, or as that found in the present findings on the basis of the lodicule types (compare the LEEE row with the LsEF row in Table I.) Moreover, where the lowest rate of hybrids was found, that is in CSGA No. 3 (5y0) and in Breeder's Seeds (293, the highest rates of seeds with a Fatua-type epiblast, 47y0 and 39y0 respectively, have been scored. The only tangible interpretation of this phenomenon lies in the very fact that these samples are not pure A. sativa; but, in fact, represent different introgressants of A. sativa with A. byzantina. This fact is known historically, and it is apparent from the morphology. The breeding history of the first sample in Table I, Harmon oats, was described in the Handbook of Canadian Varieties (1951-1967); the second sample, Garry oats by Welsh et al. (1953); and the third and fourth samples, Russell oats, by Zillinsky (1961). From

each account the extent of introgression that was created by breeding is revealed. Hence, Russell oats have more repeated crosses with varieties of "A. byzantina" origin than either Harmon oats or Garry oats. Morphologically, some characters of A. byzantina, such as length and shape of glumes, are more expressed in Russell oats than in Harmon and Garry oats. The epiblast of A. byzantina, which is also of the Fatua-type, is present at a higher rate in Russell oats (in both samples) than in Harmon and Garry oats, as seen in Table I. The presence of A. byzantina genes is the main cause of the variability of the type of epiblast in the different cul tivars.

It is worthy of mention that although A. byzantina has a Fatua-type epiblast it is not quite similar to the epiblast of A. fatua. The difference between A. sativa and A. bvzantina and other species are under investigation.

I believe that the reliability of the epiblast as a taxonomic marker should be limited to the two extreme classes when they are clear-cut, such as in the LFEF and LsEs groups of the table. The lodicule can serve, as postulated elsewhere, as a reliable marker for the determination of the species and hybrids.

Baum (1968) suggested a restricted one-way gene flow from A. fatua to A. sntiva, in discord- ance to Jain and Marshall (1967, p. 31) and Imam and Allard (1965, p. 49) among others. This suggestion was based on the fact that the combination LFEs was not found in Canadian material. The fact that A. fatua phenotypes which are cross-fertilized by pollen of A. sativa origin are eliminated by man in cultivated fields might be considered as supporting evidence for such a one-way restricted gene flow. Neverthe- less, in the present study a very limited number of the combination LFEs have been found. Among all 1200 florets observed in this study only 8 might be interpreted as LFEs (Table I), but none of these can be accuratelv determined since either the lodicule or the epiblast or both had been spoiled during the processing and preparation of the material for examination. Even if these eight examples could be regarded as LEEs type, such information would support the restricted one-way gene flow from A. fatua to A. sativa.

Taxonomically A. fatua is a well-delimited species which retains its unity and which co-

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90 CANADIAN JOURNAL OF BOTANY. VOL. 47, 1969

exists with the complex of A. sativa-byzantir~a. When looking at each morphological character separately there are many overlapping ones, but some combinations of characters are unique to each, and these are constantly maintained from generation to generation. A particular feature of A. fatua is its Fatua-type lodicule, whereas the A. sativa-byzantina complex possesses a Sativa- type lodicule. These characteristics do not change in biotypes which cannot be accurately determined on the basis of their gross morphology, for example, Thellung's "iibergangsformen", or off-types of A. sativa. The different off-types of the cultivated oats that morphologically resemble A. fatua might point to, among other factors, the genetic closeness of these species; however, they are able to retain their unity and are still distinguishable from one another by their microscopic morphology. Microscopic morphology is a very usef~il method for the taxonomy of the hexaploid oats and investiga- tions using this approach are now under way.

I want to clarify here that the circumscription of the species of oats in my monographic studies will certainly not be based solely on microscopic morphology. I am using the approach of microscopic morphology for my studies because microscopic organs play a very important role in biology of Avena. These microorgans, the lodicules and epiblasts, have an indispensable function in several important reproductive mechanisms, such as anthesis, and ontogenesis of the embryo. Their duration is very limited in comparison to that of other organs. They are well protected from the environment by the other macroorgans such as the lemmas and the glumes. The "duration factor" and the "protec- tive factor" provide these microscopic organs with less susceptibility to direct influence of the environment than they do in macroscopic organs. The characters of these microscopic organs will thus be more fully genetically expressed and less variable than those of the macroorgans whose expression is often affected considerably by various ecologic factors. Consequently microscopic morphology should be considered a basic part of taxonomic research.

Acknowledgments

I sincerely thank Miss J. Horton for technical assistance. I am greatly indebted to Mr. 0. Clayton, Canadian Seed Growers Association,

Ottawa, for providing me with the material of cultivated oats, and to Dr. V. D. Burrows, Ottawa Research Station, for providing me with material of Breeder's Seeds of Russell oats. I thank my colleagues, Drs. T. Rajhathy, R. L. Taylor, and G. A. Mulligan for review of the manuscript.

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TABORDA DE MORAIS, A. 1937. Les hybrides naturels 1953. Oat varieties, past and present. Can. Dept. d'Avena sotiva L. Bol. Soc. Broteriana 11. 12: 253-286. Agr. Publ. 891.

THELLUNG, A. 1929. Die iibegangsformen vom Wild- ZADE, A. 19120. Der Flughafer [Ave~in fatrra). Arb. hafer-typus (Avenae agrestes) zum Saathafertypus Deutschen Landw-Ges. Heft 229: Berlin. (Avenae sativae). Rec. Trav. Bot. Neerl. 25: 4 1 M . - 1912h. Die Zwischenformen vorn Flughafer

THURSTON, J. M. 1957. Morphological and physiological (Avena fatua) und Kulturhafer (Avena sativa). Fuhlings variation in wild oats (Avena fatlra L. and A. luclovi- Landw. Ztg. 61: 369-384. ciana Dur.) and in hybrids between wi!d and cultivated --- 1918. Der Hafer. Eine Monographie auf wissen- oats. J. Agr. Science, 49: 259-274. schaftlicher und prakischer Grundlage. Jena.

TSCHERMAK, E. VON. 1914. Die Verwertung der Bastardie- ZILLINSKY, F. J. 1961. Note on Russell Oats. Can. J. rung fur Phylogenetische Fragen in der Getreidegruppe. Plant. Sci. 41 : 683-684. Z. Pflanzen Zucht. 2: 208-31 1.

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the role of the lodicule and epiblast in determining natural hybrids of avena sativa ...

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