BALTIC FORESTRY NATURAL HYBRIDISATION WITHIN ELMS (ULMUS L.) IN LITHUANIA R. PETROKAS, V. BALIUCKAS Natural Hybridisation within Elms (Ulmus L.) in Lithuania RAIMUNDAS PETROKAS1 * AND VIRGILIJUS BALIUCKAS12 1 Institute of Forestry, Lithuanian Research Centre for Agriculture and Forestry, Liepu 1, LT-53101 Girionys, Kaunas distr., Lithuania 2 Aleksandras Stulginskis University, Faculty of Forestry and Ecology, Department of Forestry, Studentu 11, LT 53361 Akademija, Kaunas distr. Lithuania *Phone: 00370 37 547289, E-mail: [email protected] Petrokas, R. and Baliuckas, V. 2012. Natural Hybridisation within Elms (Ulmus L.) in Lithuania. Baltic Forestry 18(2): 237246. Abstract Putative natural hybrids (Ulmus × hollandica) between the Smooth-leaved (Ulmus minor ssp. minor) and the Wych elm (Ulmus glabra) were observed in mixed forests along the rivers and rivulets around the central part of Lithuania. Eleven populations of elms (Ulmus L.) were studied to determine 1) the critical groups of phenotypes indicative for their taxonomic identity, 2) the variability of taxa from contact zones. Three characteristics for the trees and twenty one for their leaf, including nineteen applied by WinFOLIA 2004a programme, were used to describe each sample of the fifty eight and to ascertain the degree of affinity between samples. As a result of this approach, four taxa could be distinguished at the contact zones within elms. Three elm species and hybrids, Ulmus glabra, Ulmus minor, Ulmus laevis, and Ulmus × hollandica, were fairly satisfactorily distinguishable, but there is much overlap between the Wych elm and the Field elm in their characters that can be explained only by the equivalence of their statistically discriminant phenotypic distinctness. On average, the putative natural hybrids between the Smooth-leaved and the Wych elm account for less than two percent of all individuals within group U. minor U. × hollandica U. glabra. The possibility of introgressive hybridisation is shown by the outstanding variability of morphological characteristics of the taxa from the contact zones and by the presence of extreme or novel properties of hybrids. Key words: Ulmus minor ssp. minor, Ulmus glabra, Ulmus minor, Ulmus × hollandica, Ulmus laevis, natural hybridisation Introduction Smith, Loudon, Reichenbach, Wilkomm, Schneider, Ley, and U. nitens Moench are the synonyms of U. minor The most generalised treatment of the complex ssp. minor. In Europe, this subspecies has a more taxonomical nature of European elms (Richens 1983, southerly distribution than U. glabra, and is unknown Armstrong and Sell 1996), at least in continental Eu- in Denmark, Sweden, and Norway as a wild tree, rope, considers the presence of three species in this though it is said to occur, possibly planted, on the continent, the European white elm (Ulmus laevis Pal- island of Gothland, in the Baltic (Elwes and Henry las), the Wych elm (Ulmus glabra Hudson » Ulmus 1913). Natural hybrids between the Wych elm and the montana Stokes) and the Field elm (Ulmus minor Miller Smooth-leaved elm are common in Europe, as these two sensu latissimo), the latter represented by a number species were the most widely planted in the country- of varieties, and some hybrids (Richens 1983, Mittem- side, with the former in northern areas (Scandinavia, pergher and La Porta 1991, Mackenthun 2007). Some northern Britain), and the latter further south. In ad- authors (e.g. Richens 1983) consider it a single varia- dition, the Wych elm is sometimes divided into sub- ble taxon (the species concept adopted in this study), species (Uotila 1997, Myking and Yakovlev 2006), of whereas others have proposed narrower concepts in- which the glabra (in the south of the species range) volving three species (Stace 1997), five species has leaves that are relatively broad, short tapering, (Melville 1975) to over 40 species (Armstrong 1992). with acute lobes present, trees often have a short, The Smooth-leaved elm (U. minor ssp. minor) is the forked trunk and a low, broad crown, the montana commonest type of the Field elm in continental Europe (Stokes) Lindquist (in the north of the species range today (Richens 1983, Mackenthun 2007). U. campes- northern Britain, Scandinavia) has leaves that are tris var. glabra Hartig, Planchon, Ascherson and relatively long, long tapering, without acute lobes, the Graebner, U. campestris var. laevis Spach, Planchon, upper surface of which are strigose (Sherman-Broyles U. foliaceae Gilibert, Sargent (also known as the 2007), trees commonly have a long single trunk and a Mulberry-leaved elm), U. glabra (not Hudson) Miller, tall, narrow crown. The subspecies are not accepted 2012, Vol. 18, No. 2 (35) ISSN 2029-9230 237 BALTIC FORESTRY NATURAL HYBRIDISATION WITHIN ELMS (ULMUS L.) IN LITHUANIA R. PETROKAS, V. BALIUCKAS by Flora Europaea (Tutin et al. 1968), as there is 2008). Decreasing landscape heterogeneity at all spa- much overlap between populations in these characters tial scales from microhabitats to biomes most likely and the distinction may be owing to environmental elevates the general likelihood of interspecific intro- influence, rather than genetic variation. gressive hybridization by relaxing ecological divergent Hybridization is an important mechanism for pro- selection among species and/or by removing ecolog- ducing evolutionary novelty and is considered to ical barriers to gene flow between divergently adapt- be a common mode of speciation in plants (Abbott ed species. By introgression species may acquire new 1992, Futuyma 1998). Morphological data can offer adaptive characters that allow them to colonize new actual information on hybridization related evolution- habitats or increase their fitness in their existing niche ary and ecological phenomena, because hybrids are (Minder et al. 2007). The phenomenon of hybrids hav- not always intermediate between the parental species, ing higher fitness than one or both of the parents is but often exhibit extreme and novel characters (Riese- known as hybrid vigour or heterosis. The effects of berg and Ellstrand 1993). The percentage of plants with introgressive hybridization among pure and hybrid extreme or novel characters increases in later hybrid phenotypes on a morphological variability of the taxa generations as does the percent of characters that has not yet been studied in native elms. Therefore, the show extreme. Hybrids often show characteristics that main purpose of the present study was to character- are absent from the parents (Rieseberg 1995). At least ize the phenotypic variability of native elms and to de- two independent markers are needed to demonstrate termine 1) the critical groups of phenotypes indicative that introgression is occurring, and at least one of of their taxonomic identity, 2) the variability of taxa them must be able to define the two parental species from contact zones. The novelty of the study is pre- (Harrison 1990). In the case of Ulmus laevis, a major sented by the determination of critical group of elm advantage for detailed studies is the absence of in- hybrids indicative of its taxonomic identity. terfertile species within the distribution area, remov- ing the complicating factors of hybridisation and in- Materials and methods trogression. For this reason, U. laevis was taken as datum-species in this study. Along with the Ameri- Morphological Characterization of Elm Taxa can elm (U. americana), U. laevis belongs to the Morphological characters were the main tools Blepharocarpus section which seems genetically iso- used for the identification of elm hybrids in this study. lated from the other sections of the genus (Heybroek Wych elms generally share a set of characters as fol- 1968, 1993, Townsend 1975), whereas the other two lows: stout densely villous petioles, exceeding 2-7 mm European elm species, U. glabra and U. minor, belong in length, the leaf base being strongly oblique with to the subgenus Ulmus. U. laevis is the oldest elm the lowermost lobe strongly overlapping, covering the species in Lithuania (Stockmarr 1970). petiole, leaves equally rough on the upper surface, It is customary, in sexually-reproducing organ- though rather downy beneath, the leaf margins with isms, to distinguish between species and interspecif- few acuminate lobes at a broad apex, the blade hav- ic hybrids, the latter being usually few in numbers ing 15 to 18 (20) pairs of lateral nerves, which are of- compared with the putative parent species. The genus ten forked up near the edges of the leaf, no corky wings Ulmus, however, is atypical in that, therefore the sub- on the twigs, and the absence of sucker shoots. The division of the genus into species must rest entirely Wych elm (U. montana Stokes) shows little variation on the discovery of discontinuities (Richens 1955). in the wild state, but trees growing in dense woods The approach taken here follows the lines being drawn usually bear smaller and thinner leaves than those by R. Richens (1983), who followed a fairly simple which stand in the open (Elwes and Henry 1913). The strategy. He claimed that there are just two species of common name Wych comes from Anglo Saxon mean- elms present in the British Isles, U. glabra and U. ing with pliant branches or from Gaelic meaning minor, and both species hybridise to form the natural drooping. The name glabra means smooth, a prob- hybrid elm, Ulmus × hollandica Mill. Unfortunately, able reference to the fact that the bark of this tree is every individual within the U. minor U. × holland- smoother than that of the Field elm. The common ica U. glabra group is able to hybridise again with names of the Field elm, the Smooth-leaved elm, or the one of parental species. The process may be continu- Narrow-leaved elm (see Mittempergher 1996), contrast ously repeated from generation to generation and is the smooth upper surface and narrowness of the leaves called introgressive hybridisation. It is suggested that with those of the Wych elm, which are rough and broad the most probable proximate outcome of such hybrid- (Stace 1997, White and More 2003).