a new hybrid species between two alien weeds

New habitats, new menaces: Centaurea x kleinii (C. moncktonii x C. solstitialis), a new hybrid species between two alien weeds.Anthropic alteration of habitat opens the door to the cohabitation of imported species that in their native range would never coexist. Centaurea solstitialis and Centaurea moncktonii are two invasive species whose natural distributions in Eurasia do not overlap. After introduction in North America, invasive spread led to overlapping distributions. Although the environmental requirements differ between the two species, the close proximity of diverse habitats (within pollinator range) has resulted in several cases of natural hybridization between the two. The result of the cross between these two distantly related species is a sterile perennial. Many of its characteristics are intermediate between its parents, but morphologically it is closer to Centaurea moncktonii (probably the maternal parent, and itself of hybrid origin). The plant could possibly become an invasive weed through clonal reproduction. The apparent maternal parent, which the hybrid may more closely resemble physiologically as well as morphologically, is a wellknown creeping weed in alpine pastures throughout Europe.


INTRODUCTION
Worldwide plant migration has been greatly accelerated by human activities, especially farming and grazing of domestic livestock.One of the oldest examples is Centaurea cyanus L., associated with the cultivation of cereal grains since Neolithic times (Rösch, 1998).Although the development of herbicides reduced the impact of C. cyanus on crop yields, other species of Centaurea rank among the worst noncropland weeds in western North America: C. solstitialis L., C. diffusa Lam., C. stoebe L. (= C. maculosa Lam.), and C. moncktonii C.E. Britton (= C. pratensis Thuill. 1 ) (Roché & Roché, 1991a).
Invasion of new regions has brought species together that formerly were seldom (if ever) in close proximity.Some combinations (e.g., C. jacea L. x C. nigra L.) have long been widely known in Europe, and introduced in North America as hybrid swarms (more detail below).Other combinations believed until recently to be new (e.g., C. stoebe x C. diffusa = C. x psammogena Gáyer) were previously reported from their native ranges (Gáyer (1909) in Marrs et al., 2006;Ochsmann, 2000).Recent research suggests that the multiple introductions of C. diffusa included "admixed individuals" and that the hybrids were not newly created post-introduction (Blair & Hufbauer, 2009).Other hybrids are entirely unexpected, because in addition to geographic separation, the taxa are only distantly related.That is the case with an apparent hybrid discovered in 1999 in southern Oregon, where C. moncktonii and C. solstitialis grow in close proximity (Fig. 1).Despite the improbability of this hybridization, some individuals were spotted as obvious intermediates between these two species.
According to Garcia-Jacas et al. (2006), both putative parents belong to the Centaurea Jacea clade that stands as the core group of Centaurea.However, in a geographic classification system, they segregate into two different clades.Centaurea moncktonii belongs to section Jacea, which is in the clade of widely distributed species.Centaurea solstitialis belongs to section Mesocentron among the Western Mediterranean group of species (Garcia-Jacas et al., 2006).Thus, despite sharing the important characters that define the Centaurea Jacea clade (pollen type Jacea and achene with a double pappus and lateral hilum), they are only distant relatives.
Based on multiple chromosome counts (Watanabe, 2010), C. moncktonii is a tetraploid with 2n = 4x = 44 (as C. pratensis), while C. solstitialis is diploid with 2n = 16.Probability of viable offspring from this cross should be extremely low.The apparent hybrid has been found in four separate locations (Fig. 1); the distance and lack of seed production indicate that the hybrids arose from independent crossings.Morphologically, the hybrid is similar to C. moncktonii, but with obvious traces of its C. solstitialis ancestry in the phyllary appendages, floret color, and indumentum of the vegetative parts.
1 Centaurea pratensis Thuill.and C. debeauxii Gren.& Godr.are among previous names widely used for the hybrid between C. nigra L. and C. jacea L., but according to Wagenitz (1980), the correct name for this species is C. moncktonii C. E. Britton.

Pollination
Centaurea species in general are entomophilous (insect pollinated) and are visited by a wide range of species for pollen and/or nectar (Marsden-Jones & Turrill, 1954).The disc florets containing the functional gynoecium and androecium are actinomorphic and gamopetalous; an insect visiting a capitulum during anthesis will push its proboscis successively into florets with open corollas.To collect pollen and nectar, an insect must deal with every floret separately, but it can pollinate many florets by crawling a very short distance, or by simply turning.Each floret has only one ovule requiring only one viable and compatible pollen grain to bring about fertilization (Marsden-Jones & Turrill, 1954).

Geographic distribution
Centaurea solstitialis has a wide distribution along the Mediterranean region in its broadest sense, from Iran and Turkey to the Iberian Peninsula and North Africa.It originated in the Eastern Mediterranean, but has expanded to the entire Mediterranean region, usually associated with anthropic disturbance.As an alien invader, it has colonized extensive areas in the western USA, South America and Australia.Centaurea solstitialis became established in California in the mid-1800s and by 2000 infested more than four million hectares (Thomsen et al., 1996).It was first reported in Oregon in Douglas County in 1914 (Roché & Talbott, 1986).
Centaurea moncktonii is a frequent and fully fertile hybrid between C. nigra and C. jacea.Centaurea nigra is native to the British Isles, with hybrids being common when C. jacea was introduced from the European Continent (Marsden-Jones & Turrill, 1954).Ockendon (1975) described the reverse situation in France and suggested that C. jacea is native and C. nigra introduced, which is much disputable following Dostál (1976).Because the F1 hybrid can back-cross with either parent and with other F1 individuals, hybrid swarms normally completely replace the pure forms.Both the parent species and the hybrid are common in Eurasia, becoming rare to the East.
The earliest reports of C. moncktonii in Oregon date to 1911 and 1918 in the Willamette Valley from Portland to Eugene (Roché & Talbott, 1986).Howell (1959) reported that meadow knapweed (C.moncktonii) was cultivated for winter forage near Roseburg (Douglas County), where it remains abundant today.It is leafier and more palatable to livestock than other knapweeds, but as plants mature, both palatability and nutritional value decline (Roché & Roché, 1991b).

Ecological distribution
In its native range, C. soltitialis seldom grows in the mountains, being restricted to the Mediterranean climate of its origin.In western North America, its distribution is also correlated to Mediterranean climates, with the most aggressive invasion in California.In the more northern latitudes (e.g., Washington State, ca.46º to 49º North), it is restricted to sunny, warm habitats such as south slopes at lower elevations, and despite repeated introductions, does not persist in British Columbia, Canada (Roché & Thill, 2001).In Oregon, it is most competitive in the Mediterranean climate of the inland valleys between the Cascade and Coast Range mountains (a small, northern extension of the Central Valley of California), the warm basalt slopes in the Columbia Gorge, and south slopes in the foothills of the Blue Mountains.
According to most of the monographers (e.g., Marsden-Jones & Turrill, 1954;Wagenitz, 1987), C. moncktonii is a meadow plant and in the Mediterranean region it grows only in the moistest places in the mountains.In western North America, C. moncktonii extends from coastal northern California through Oregon and Washington into British Columbia, primarily in moist forested regions and in meadows, irrigated pastures, and riparian zones (Roché & Johnson, 2003).The largest populations of C. moncktonii in Oregon are in the interior valley lowlands (Umpqua and Willamette Valleys) including both the east slopes of the Coast Range and the west slopes of the Cascade Range.In Douglas County the Klamath Range forms a mountain bridge between the Coast and Cascade Ranges; annual precipitation ranges from 840 mm in the lowland valleys to over 1300 mm in the foothills (Johnson et al., 2003).Substantial populations also grow on the mesic eastern slopes and valleys of the Cascade Range near Hood River in the Columbia River Gorge in northern Oregon.
Contact between C. moncktonii and C. solstitialis within the range of their native distribution is infrequent because their ecological requirements differ so greatly.What differs in southern Oregon is not the biology of the plants, but that appropriate habitats occur within flight range of their pollinators.Both species share disturbed sites on roadsides that are adjacent to mesic meadows and riparian zones that support C. moncktonii and warm south-facing slopes that are dominated by C. solstitialis.
The new species is intermediate in most of its characters between C. solstitialis (Figs.3A, D) and C. moncktonii (Figs.3C, F), but the habit is more similar to C. moncktonii.

Figure 2 .
Figure 2. Drawings of the capitulum and detail of the bracts (from outermost to innermost) of Centaurea solstitialis (A and D), C. x kleinii (B and E) and C. moncktonii (C and F).Scale: 5 mm in all the cases.Drawings by Carles Puche.