The herbaceous vine, twisted birthwort Aristolochia contorta Bunge, is a rare species listed in the Red Data Book of Primorsky krai (2008). On the northern boundary of its range (south of the Russian Far East), the species is represented by small isolated populations confined to the river drainages. Using allozyme analysis, genetic variation of nine natural populations of A. contorta (247 accessions), which represented the main part of the species range in Russia, was examined. The values of genetic variation indices (P = 22.7%; A = 1.28; H o = 0.129; H e = 0.101) were low and comparable with the data obtained for other rare plants. The proportion of unique genotypes (G/N) and Simpson's genotypic diversity index (D) ranged from 0.32 to 0.64 and from 0.60 to 0.98, respectively. This means that A. contorta is characterized by sexual and asexual reproduction. Moreover, the ratio between these types of reproduction varied among the populations. Complete absence of inbreeding and excess of heterozygotes (F IS = -0.282), which was low probable in case of free mating, was observed. Evidently, clonal growth and (or) apomixis enables the species to maintain certain level of heterozy gosity despite of small population sizes and non regular gene exchange.

Pollen ultrastructure has been studied in two relict and rare species of the genus Aristolochia, A. contorta Bunge and A. manshuriensis Kom. (Aristolochiaceae). Both species have inaperturate, spheroidal, sometimes distally monocolpate or distally bicolpate pollen grains. The equatorial and polar axes of pollen grain in A. manshuriensis are 48.5 and 44.0 μm, respectively. The percentage of defective pollen grains in A. manshuriensis is 3.4%. The fossulate, perforated exine is up to 2.3 μm in thickness; the sexine and the nexine are almost equal in thickness. In A. contorta, the equatorial axis of pollen grain is 36.6 μm: the defectiveness percentage, 24.5%. The exine is verrucate, up to 0.3 μm in thickness, while the sexine is two to three times thicker than the nexine. The pollen germination experiments have shown that pollen of A. manshuriensis, in contrast to A. contorta, can germinate in 10-20% sucrose at 22°С. These data and the high percentage of pollen defectiveness in A. contorta indicate that the androecium function in this species is reduced. The reduction of the androecium function is evidenced by a small amount of pollen grains in anthers or empty anthers and a high percentage of defective pollen grains.

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