Allium occultum Tzanoudakis & Trigas, 2015

Tzanoudakis, Dimitris & Trigas, Panayiotis, 2015, Allium occultum, a new species of A. sect. Codonoprasum (Amaryllidaceae) from Skiros Island (W Aegean, Greece), Phytotaxa 202 (2), pp. 135-142 : 136-140

Allium occultum Tzanoudakis & Trigas , sp. nov. ( Fig. 1 )

It differs from the related Allium apergii in tepals and anthers colour, fewer flowers, smaller spathe valves, larger ovary and capsule, papillose-denticulate leaf margins, yellowish-brown to brown outer bulb coats and light to dark purple outer scale leaf.

Type:— GREECE. Western Aegean Islands: Skiros Island, Mt. Kochilas, rocky calcareous slopes south of the summit area, phrygana with Sarcopoterium spinosum and Thymbra capitata , 590 m, 38 o 49.183’ N, 24 o 36.547’ E, 10 July 2014, Trigas 5812 (holotype, ACA!, isotypes, UPA!, ATHU!).

Bulbous perennial herb. Bulb ovoid to narrowly ovoid, 7–12 × 5–8 mm, bulbilliferous, with outer tunics coriaceous to papery, yellowish-brown to brown, the inner ones membranous, whitish to purple; outer scale leaf light to dark purple. Bulblets attached to the main bulb. Stem 10–20 cm high, erect, glabrous, 0.8–1.2 mm in diameter, covered by leaf sheaths for 1/3–2/3 of its length. Leaves 2–4, semicylindrical, ribbed, papillose-denticulate at margins and ribs, 6–11 cm long and 0.5–1 mm wide. Spathe persistent, with two subequal to unequal, erect to spreading valves, the larger 12–40 mm long, 5–7-nerved, and the smaller 6–23 mm long, 3–5-nerved. Inflorescence subhemisphaeric, ± compact, 5–12-flowered; pedicels 3–7 mm long in flower, up to 18 mm long in fruit. Perigone campanulate; Perigone segments subequal, narrowly elliptical, 4.5–5 × 1.6–2.2 mm, concave, rounded, apiculate, uniformly purplish-pink to pinkishwhite, with a darker purplish-brown to purple mid-vein. Stamens included, slightly exserted at late flowering, with simple and slender, white to purple filaments 3.5–4.0 mm long, connected below into an annulus c. 1 mm long; anthers cream, oblong-elliptical, c. 0.8 × 0.6 mm, rounded at the apex. Ovary obovoid-cylindrical, 2.8–3 × 1.6–2 mm, sessile, narrowed at base, truncate and papillose above, whitish from the base to the middle and greenish-yellow to orange at the apex. Style white. Capsule widely obovoid to subglobose, 3.5–4 × 3.5–4 mm.

Etymology:—The specific epithet ( “ occultum ”, i.e. hidden) comes after the peculiar ecology of the new species, which usually grows under the protection of cushion-like shrubs, protected from herbivore animals.

Karyology:—In the investigated material the chromosome number 2n = 32 was found ( Figs. 2 , 3 ). Considering that the basic chromosome number x = 8 characterizes all Greek species of Allium sect. Codonoprasum ( Tzanoudakis & Vosa 1988; Tzanoudakis 1992; Brullo et al. 2001, 2003, 2010; Biel et al. 2006, Trigas et al. 2010), a tetraploid level for the new species is suggested. It is worth noting, however, that in spite of this tetraploid chromosome number, according to the karyotype structure of all individuals studied, the new species seems functionally diploid. In all metaphase plates two sets of chromosomes are easily recognized: 6 evidently anisobrachial chromosomes and 26 more or less isobrachial ones ( Fig. 2 ). According to the available literature (see above), most Greek species of A. section Codonoprasum are characterized by karyotypes consisting of either exclusively metacentric chromosomes or, in cases that anisobrachial chromosomes are present, they belong to the submetacentric type (sm). Only in the recently described tetraploid A. apergii from Euboea Island a set of 12 anisobrachial (4sm + 4sm/st + 4st) chromosomes has been observed ( Trigas et al. 2010). The karyotype diploidization of the new species is further supported by the second set of ± isobrachial chromosomes. Their number itself (26), does not recall tetraploidy and the presence of a pair of metacentric chromosomes evidently smaller than others also support the diploidization of the chromosome complement. The six subtelocentric chromosomes in its tetraploid complement refer either to allopolyploid origin (e.g. combining one diploid set with two subtelocentric chromosomes and one with four such chromosomes) or, less likely, to chromosomal translocation. The karyotype of A. occultum is also characterized by a) the absence of nucleolar organizers (SAT) chromosomes from the metaphase plates and b) the presence of 2–3 small B-chromosomes observed in some individuals of the type population ( Fig. 3 ).

Distribution:— Allium occultum is currently known from two neighbouring localities in the SE part of Skiros Island ( Fig. 4). The type locality lies S-SE of the summit area of Mt. Kochilas, at the edge of a small plateau covered by low spiny vegetation. Some individuals were located in the small valley along the path that leads to Mt. Kochilas, just below the type locality. The second locality lies c. 1 km map distance S-SE of the type locality, along an eastwards directed ridge.

Ecology:— Allium occultum grows on rocky calcareous slopes covered by phrygana and in the understorey of degraded evergreen scrubland. The soil is thin and infertile and usually limited to small patches between the rocks. The whole area is exposed to strong north winds, especially during the summer. The accompanying taxa of A. occultum are typical members of the phrygana communities in the Aegean, such as Thymbra capitata ( Linnaeus 1753: 568) Cavanilles (1803: 37) , Sarcopoterium spinosum ( Linnaeus 1753: 994) Spach (1846: 43) , Euphorbia acanthothamnos Heldr. & Sart. ex Boissier (1859: 86) , Ballota acetabulosa ( Linnaeus 1753: 584) Bentham (1834: 595) , Fumana arabica ( Linnaeus 1755: 14) Spach (1836: 359) , Brachypodium distachyon ( Linnaeus 1756: 8) Beauvois (1812: 101) , Hyparrhenia hirta ( Linnaeus 1753: 1046) Stapf (1918: 315) , Sideritis curvidens Stapf (1885: 100) . The evergreen scrublands, where A. occultum grows, consist mainly of Quercus coccifera Linnaeus (1753: 995) , Phillyrea latifolia Linnaeus (1753: 8) and Acer sempervirens Linnaeus (1767: 128) .

Allium occultum is restricted to the upper elevational zone of Skiros Island (520–650 m a.s.l.) and flowers in July. Flowering individuals are almost exclusively observed to grow in cushion-like spiny shrubs, where they are protected from grazing. They usually grow alone or in small groups of 2–3 individuals.

Additional specimens seen (paratypes):— Skiros Island , Mt.Kochilas, ca. 600 m a.s.l., 24June 1996, Tzanoudakis, Trigas & Samaras 13888 ( UPA!) ; Tz 97/119 cultivated (UPA!).

Taxonomic relationships:—Based on the absence of conspicuous nectaries at the base of the ovary, the simple filaments and the two opposite and unequal spathe valves, with at least one of them longer than the pedicels, Allium occultum undoubtedly belongs to A. sect. Codonoprasum . The campanulate perigone and the included stamens place it in the complex A. paniculatum Linnaeus (1759: 978) group. Several species of this group are distributed in the Aegean Islands and the adjacent mainland. Allium occultum , however, is well distinguished from all species of this group by a combination of several morphological characters.

Allium occultum should be added to a group of tetraploid species, members of A. sect. Codonoprasum , recently described from the Aegean area: A. apolloniensis Biel et al. (2006: 367) , A. dirphianum Brullo et al. (2003: 133) and A. apergii Trigas et al. (2010: 226) . Allium apergii is probably the closest related species to A. occultum . The two species, however, are clearly distinct by several morphological characters ( Table 1). Allium dirphianum is a more robust species, which mainly differs from A. occultum by the absence of bulbets, its yellowish and slightly coriaceous inner bulb coats, larger spathe valves, shorter filaments, apiculate anthers and larger perigone segments, ovary and capsule. Allium apolloniensis markedly differs from A. occultum by its narrowly campanulate perigone with white to pale pink tepals, dark reddish purple anthers, capitate stigma, bulblets produced at the above ground part of the scape, the autumnal flowering period and the autotetraploid chromosome arrangement.

The uniformly coloured purplish-pink tepals of A. occultum are found in some W Aegean Allium species, but they are all members of A. sect Scorodon Koch (1837: 716), i.e. A. runemarkii Trigas & Tzanoudakis (2000: 89) , A. calamarophilon Phitos & Tzanoudakis (1981: 11) and A. thessalicum Brullo et al. (1994: 183) . This feature is relatively rare among the Aegean members of A. sect. Codonoprasum ; only the East Aegean A. candargyi Karavokyrou & Tzanoudakis (1994: 142) and A. exile Boiss. & Orph. in Boissier (1859: 118), as well as the C Aegean A. brulloi Salmeri (1998: 71) and A. pilosum Sm. in Sibthorp & Smith (1809: 225) show this feature.All these species largely differ from A. occultum in several morphological characters, including, among others, shape of the ovary ( A. candargyi ), perigone shape and size ( A. exile , A. brulloi ) and indumentums ( A. pilosum ). Additionally, A. occultum is tetraploid, while all the above species are diploids and they cannot be considered as closely related to it.

The close affinity of A. occultum and A. apergii is further suggested by their karyotypes and especially by the presence of the subtelocentric chromosomes in their chromosome complements. The number of subtelocentric chromosomes of A. occultum , however, differs from that of A. apergii , as from that expected in a tetraploid karyotype, indicating the interpretation problems raised regarding diversification and evolution patterns of the genus Allium in the Aegean area. The diversification patterns of the genus are particularly intricate in western Aegean Islands, which are located close to the Greek mainland. There exists a transition zone occupied by both, island and mainland floristic elements. Diploidization phenomena of the chromosome complement has already been observed in other polyploid members of A. sect. Codonoprasum in Greece, i.e. A. dirphianum ( Brullo et al. 2003) , A. dentiferum Webb & Berthelot (1848: 345), A. pallens Linnaeus (1762: 427) and A. guicciardii Heldreich (1876: 233) , but it is observed so clearly for the first time in A. occultum .

Conservation:—The whole Mt. Kochilas area is isolated within Skiros Island, without any road access. This inaccessible region is very difficult to be explored thoroughly and A. occultum has only been located in two neighbouring peaks that are only accessible by a few hours walk. It is likely that the species has a wider distribution range within this area of the island, because its habitat is common throughout the entire region. The species is rare, however, and only few groups of individuals have been observed so far, each of them consisting of about 10–20 plants growing scattered under the protection of spiny shrubs. From our field observations, it is estimated that the total population of A. occultum should count less than 1,000 individuals.

The main threat that Allium occultum faces is overgrazing, which has significantly affected the vegetation cover of Mt. Kochilas. Approximately 40,000 goats graze free in Skiros Island, most of them in its SE part. Thus, the vast majority of A. occultum individuals were observed to grow under spiny shrubs, protected from the goats. Its bulb grows very close to the soil surface and consumption of stems and leaves often leads to eradication of the bulb and subsequently to its destruction.

Mt. Kochilas is part of the Natura 2000 network of protected areas in Greece (GR2420006) and it has been characterized as Site of Community Importance (SCI), Special Protection Area (SPA) and Special Area of Conservation (SAC). This has not prevented to make plans for a large wind farm created in this region. The installation of wind generators along the ridges of the region, together with the infrastructures required for their transportation and installation, will certainly lead to a significant population decline of A. occultum .

Figures

FIGURE 1. Allium occultum sp. nov. in its natural habitat. (A) flowering individual growing protected in Sarcopoterium spinosum, (B) inflorescence, (C) flower detail, (D) inner bulb tunics and outer scale leaf, (E) outer bulb tunics and (F) the natural habitat of A. occultum.

FIGURE 2. Karyogram of A. occultum, 2n = 4x = 32.

FIGURE 3. Metaphase plate of A. occultum, 2n = 4x = 32. The long arrows indicate the subtelocentric chromosomes and the short arrows the B-chromosomes.