||Systematical approaches to the genus Avena L.
|Dr. Igor Loskutov
This article presents some historical aspects of the inter- and intra-specific classification of the genus
Avena L. The main investigations of oat taxonomists are mentioned. The systematic position of the species in the genus are discussed. The position of hull-less oats in the system of species is described. Keys for the identification of the species are given. The main taxonomic characteristics for identification of species in inter- and intra-specific classification are presented. The author has presented own point of view for systematic position and intra-specific classification of wild species of genus Avena L.
Classification, based primarily on morphological traits is the fundamental basis for botanical research, preservation and use of the global plant genetic resources collections. There is no agreement among researchers regarding the systematics of the species in Avena L. The history of systematics of this genus started about three hundred years ago. Linnaeus described four Avena species, Avena sativa, A. fatua, A. sterilis and A. nuda (Linneaus, 1753; 1762). Among the numerous publications of the 19th century dedicated to the systematics of Avena L., the most significant taxonomic surveys were by Marshall and Bieberstein (1819), Grisebach (1844), Koch (1848) and Cosson and Durie de Maisonneuve (1855). The classification of Cosson and Durie de Maisonnueve was the most detailed of the period. These systematic surveys were based on such traits as the shape of the panicle, character of rachilla and lemma pubescence, as well as, the character of disarticulation of spikelet florets. Initial classifications did not reflect phylogeny between species. However, they were founded on detailed morphological descriptions of various oat species and producing an integral system of this genus. Natural or phylogenetic classifications that outlined groups of related species and derived cultivated species from wild species were developed later (Jessen, 1863; Haussknecht, 1885; Trabut, 1909; Thellung, 1919, 1928). Haussknecht (1885) established that hexaploid cultivated species had originated not from one, but at least from three or several wild species. Similar views on the polyphyletic origin of oats were shared by other researchers (Thellung, 1911; Trabut, 1914; Zade, 1918).
I. Interspecific classification.
In 1880, Russian botanist Regel (1880) developed a classification of Avena based on rachilla and
lemma tip shape, and lemma pubescence and the disarticulation of florets. These traits remain the key traits in Avena L. classification.
The most detailed classification of this genus was developed by Malzev (1930) and remains the most cited Avena monograph. Many assumptions concerning the position of a number of species made by Malzev on the basis of analysis of a complex of morphological and biological characteristics were later confirmed by genetic, karyology and cytogenetic data (Rajhathy & Thomas, 1974). Malzev produced the most comprehensive phylogenetic system of a typical section of this genus (Euavena - Grisebach, 1844) based on lemma tip characteristics by dividing it into two subsections, Aristulatae Malz. and Denticulatae Malz., and by analysing all wild species known at that time (Table 1). According to Malzev (1930), Avena includes 7 species represented by 22 subspecies and 184 recognizable groups within subspecies (varieties and forms). The work of Malzev drew heavily on that of Thellung (1928). Malzev made a thorough analysis of Avena based on the major classification principles dominating at that time, where subspecies was an important taxonomic unit.
Malzev (1930) considered that A.strigosa Schreb. consisted of a number of wild and cultivated subspecies with different ploidy levels. Malzev considered the diploid species A. strigosa subsp. hirtula (Lag.) Malz. was the progenitor of cultivated A. strigosa subsp. strigosa (Schreb.) Thell., which included prol. brevis (Roth) Thell. and prol. nuda (L.) Hausskn.
The wild tetraploid subspecies A.strigosa subsp. vaviloviana Malz. was considered the progenitor of cultivated A.strigosa subsp. abyssinica(Hochst.) Thell. The hexaploid species A. fatua L. and A. sterilisL., the concept of which was considerably expanded by the author, were divided into 8 and 6 subspecies respectively half of the subspecies being wild and half cultivated. Each subspecies had distinct areas of distribution and differed in a number of morphological characters. Further development of a
natural classification for the genus resulted from explorations of Vavilov (1919, 1926, 1927), Nevsky (1934) and Mordvinkina (1936). For detailed structuring systems of the genus the authors used morphological characteristics and plant immunological data for species and subspecific taxa, genetic and cytological data, and new information on the areas of distribution for each species. In the mid 20th century several new classifications of Avena L. were developed, for example, by Sampson (1954), Stanton (1955), Mansfeld (1958), and Coffman (1961). Recent literature has not critically examined the taxonomic attribution of the oat species presented by Malzev (1930) and Vavilov (1927). Publications on the classification of Avena L., especially on its cultivated species, in the Russian literature since the monograph of Malzev
are little known outside Russia (Petropavlovsky, 1931; Mordvinkina, 1936, 1969; Roshevitz, 1934; Rodionova et al., 1994).
Using the cultivated oat species in the germplasm collection of VIR (Russia), Mordvinkina (1936)
developed a natural taxonomic system of Avena L. based on the plant immunology data and the areas of distribution outlined by Vavilov (1926), determined relationships among species and related this to the work of Malzev (1929). She transferred all hexaploid subspecies to the rank of species: A.sativa L., A. byzantina Koch, A. fatua L. and A.sterilis L. She recognised diploid and tetraploid groups as species. This classification became widely accepted because it represented natural
combination of the contribution of Malzev’s (1930) specific and Koernike’s (1885) intra-specific polymorphism of cultivated species and other scientists. From practical point of view, the main value of this system was the consistent relationship established between the cultivated and wild forms within the genus Avena.
Monographs on many crops have been published by staff of VIR. Since 1935 about 30 monographs of "Cultivated Flora" have been published (Loskutov, 1999). Systematics of Avena L. is represented in two monographs (Mordvinkina, 1936; Rodionova et al., 1994).
Systematics of the genus Avena was accompanied by specification of the taxonomic position of all naked oat forms which had previously been classified into a separate species (Mordvinkina, 1936). The form described by Linnaeus as A. nuda was reclassified as a subspecies of A. strigosa (Haussknecht, 1899). Mordvinkina (1936) reserved for this form the rank of series – proles nuda (L.) Haussk. within the diploid cultivated species A.strigosa Schreb. of European origin. Karyological and genetic data proved that such forms may not be regarded as independent species because they have the same karyotype as A.strigosa and are sympatric (Nikolaeva, 1922; Mordvinkina, 1929). The results of experiments showed that naked diploid forms of A. strigosa do not cross with hexaploid species A. sativa including the naked forms of Asiatic origin, which is evidence of their different genetic nature. At the same time, naked diploid forms are easily crossed with hulled ones of A.brevis Roth. and A. strigosa Schreb. and possess immunity against parasitic fungi, thus being completely different in this feature from hexaploid naked forms (Vavilov, 1919). Naked diploid forms of A. strigosa differ from hulled ones of this species only by one gene responsible for the multiflorous spikelet effect (Zhegalov, 1924; Emme & Mordvinkina, 1939). The same situation occurs in hexaploid forms of naked oats (A. sativa subsp. nudisativa (Husnot.) Rod. et Sold.). Alefeld (1866) was the first to divide species A. sativa into groups. He named naked forms A. sativa nuda Alef. and subdivided them into several varieties. It was shown that the greatest polymorphism of characters (botanical varieties) in naked hexaploid oats had been traced to China, and those forms were introduced to Europe (Vavilov, 1926). It is reported that in the 5th century A.D. oat forms cultivated in China were represented exclusively by hexaploid naked
oats (Zhukovsky, 1971). Unfortunately, due to the gaps in our knowledge, there is still a debate regarding the ploidy level and taxonomic position of hypothetical species A. nuda L., most of non-Russian authors have been named as the diploid species A.strigosa and A.nuda as synonyms and some of them said about (botanical variety) var. nuda for A.sativa.
Two tendencies are obvious in the modern classifications of Avena: an increase in the number of
species and subdivisions of species into smaller ones on the basis of morphological and especially karyological differences. Extreme examples of these trends are such classifications where the number of Avena species is either only 7 (Ladizinsky & Zohary, 1971) and 14 (Ladizinsky, 1989), or
34 (Baum, 1977). In the first case, one can see unjustified consolidation of species based only on karyological data disregarding their areas of distribution. In the second case, the rank of species is attributed to hybrid species, forms and mutants, which happens quite frequently. Both tendencies are
factors that hamper practical application of such taxonomic systems.
The system of Mordvinkina (1936) supplemented by genetic, karyological and morphological data with the
addition of newly described species served as a foundation for the development of a new classification of Avena L. (Rodionova et al., 1994). Keys for the identification of all species within this genus together with accepted additions (Loskutov, 1998b) are presented (Table 2).
II. Intra-specific classification.
Complexity of a species as an integral unit consisting of various forms and genotypes was emphasized by
Vavilov (1931) and he used Avena as an example for discussing species complexity. In view of this, natural intra-specific classification should, in our opinion, be based on a complex approach to the concept of the rank of botanical variety as an objective unit in complicated polymorph specific systems.
Currently there are few descriptions of the patterns of intra-specific classification in oats. The
first such system of Avena L. was that of Alefeld (1866). He described 15 botanical varieties of cultivated species A. sativa L.; some of them were used by other authors as smaller taxonomic units (Seringe, 1819; Krause, 1837; Metzger, 1824). Later many of these taxa were included in further
classifications as botanical varieties (Koernicke & Werner, 1885; Trabut, 1909; Marquand, 1922; Thellung, 1928). The most comprehensive intra-specific system was developed by Koernike(1885) and was adopted in Russia by others (Flyaksberger, 1908; Flyaksberger at al., 1939). Unfortunately after the above-mentioned authors further development and, what is more important, practical utilisation of intra-specific classifications of Avena L. in Europe came to an end. This situation was primarily associated with the natural fact that the great diversity of primitive local varieties and populations of cultivated oats available in Europe in the beginning of this century was irrevocably lost in the 1940’s and 1950’s when new unified developed breeding
cultivars, mainly from America, replaced many European landraces.
Intra-specific taxonomic systems of Avena L. currently existing and widely used in Russia are
rarely cited outside Russia (in English publications especially). At present Mordvinkina’s classification (1936) with certain amendments (Rodionova et al., 1994) is used for cultivated oat species (Tables 3-5). It is based on the principles and approaches developed by Alefeld (1866), Koernike (1885) and other taxonomists who studied intra-specific polymorphism of Avena species. Intra-specific taxa have been recognised for four cultivated oat species, namely: A. sativa L., A. byzantina Koch, A. abyssinica Hochst. and A. strigosa Schreb. The foundation of this classification was based on clearly discernible morphological characters, such as shape of the panicle, colour and pubescence of lemma, length of glumes,
awnedness, character of disarticulation of florets in a spikelet, and character of the karyopsis (naked or hulled).
The value of these intra-specific classification systems is that they enabled identity and, consequently, preservation and study of the whole diversity of each oat species. In the former USSR, at VIR’s initiative, the taxon of botanical variety has been used since 1930’s as the official standard in works associated
with the release of cultivars and commercial seed production (Rukovodstvo, 1938).
In the last decades, Avena wild species collection have been replenished by new accessions and newly described species from all regions of Mediterranean and Black Seas. Presently VIR stores an oat germplasm collection of 2000 accessions including 22 wild species, that comprehensively reflects the whole spectre of intra- and interspecific diversity of Avena L. (Loskutov, 1993; 1998a). Increased
volume of plant materials in storage, representing wide variability of different characters, calls for critical reviewing of the existing systems and for analysing taxonomically significant characters in order to update, and provide a practical system for taxon identification within the genus Avena.
A revised taxonomic system for the species in the genus Avena based the degree of relationship with
hexaploid species has been developed (Loskutov, 1999b). This system incorporates the information obtained during an analysis of the world-wide scientific literature and the results comprehensive morphological and
biological analysis, including data on karyotype, electrophoretic spectra of avenin storage proteins and RAPD markers analysis (Table 6).
Based on the law of homologous series in variation (Vavilov, 1922) a new itra-specific classification system (Table 7) was developed combining information from both prior taxonomic systems and recent
morphological studies (Loskutov, 1998c).
While studying the diversity of wild oat species, the greatest amplitude of variability of characters
typical for all species, on the one hand, and environmentally stability of their manifestation, were recorded for karyopsis traits. Among these traits were the colour of lemma, character and colour of pubescence, which had been repeatedly noted by Malzev (1930) and Vavilov (1922) during their studies. These traits are most useful in Avena classification. These traits are also of primary importance for identification of lower taxonomic ranks within a species. Initially, each of these traits was assessed using the International
Descriptors for the genus Avena L.(1984). Development of an intra-specific classification was based on characters of the karyopsis. Identification of accessions in accordance with the system described here
should help improve conservation of the diversity of wild oat species.
Inter-specific and, especially, intra-specific classification presently becomes more and more important not only in botanical, breeding and plant science research, and especially in genebank activities related to preservation of plant seed diversity (Loskutov, 2000), but also in genetic investigations involving
molecular technologies (RLFP, RAPD, etc). Any research on the level of entire plant or separate cell organelle should be started with detailed identification of the studied material from botanical point of view. If such material has been incorrectly identified with inaccuracies in attributing it to a certain species, the results of such research could be erroneously transferred onto other closely related species and genera, which may lead to a wrong interpretation of final results and mistakes in formulation of conclusions. Thus, development
and, primarily, practical application of complex polytype systems of factual polymorphism within a species and within the entire genus provide for better understanding, more detailed study and safer conservation of the whole global diversity of plant genetic resources for future generations.
I am grateful to my colleague Dr. T.N. Smekalova (VIR) for making valuable suggestions and I acknowledge
critical reading of this manuscript by Dr. D.A. Vaughan, NIAR, Japan.
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| Systematical approaches to the genus Avena L.