|其他摘要||Alcimandra cathcartii Dandy (Magnoliaceae) was appraised the “First Grade” status of the Chinese wild endangered plants that deserves strict protection. Since 2004, it has been listed in the China Species Red List as a category of “EN A 2c” species. Based on the IUCN criteria, A. cathcartii in China has endured persistent threat during the last three generations and has lost at least 50% of its individuals. According to our field observations, the populations are dominated by secondary and weak individuals (e.g., in the population of Heizhiguo Township, Guangnan County in Yunnan Province). In the worst case, a population in Gongpinghe Township, Jingdong County has been totally destroyed and becomes locally extinct there. To make clear the living status of A. cathcartii and to explore its genetic background, we investigated its breeding system and genetic diversity through AFLP marker. Ecological and biological characteristics, chromosome type of different populations, floral and pollination biology and historical events were also studied and analyzed. Finally, a conclusion was made in terms of living status and the cause based on the synthetical data. Some suggestions on protecting and expanding populations were also provided. The major results were as follows:
1. Ecological, biological characteristics and chromosome traits
Alcimandra cathcartii distributes sporadically and often grows in sunny places. The fruits are aggregated often bearing inviable seeds. The seeds with arils rich in oil often had themselves eaten and taken away by birds and squirrels. The fresh seeds show very high variability (>90%), whereas they are physiologically dormant, the seedling productivity was very low under natural conditions.
Alcimandra cathcartii was found for the first time that it shows differences in karyotypes among different populations mainly presented by the types of chromosome and the occurrence or non-occurrence of microsatellites. For example, the karyotype of JP population can be formulated as 2 n = 2 x = 38 = 2 M + 22 m+ 14 sm, whereas the PB population is 2 n = 2 x = 38 = 22 m + 15 sm (3sat) + 1 st. This may be related to the varied geographic locations of populations.
2. Floral phenology, pollination biology and breeding system characteristics
Our study indicates for the first time that the flowers of Alcimandra cathcartii keep unclosed once opening. The gynoecia and stamens often mature simultaneously, and sometimes the anthers dehisce a little later than the petals opening. The stigma without nectar secretion has three-day acceptability or so which occurs before flower opening.
The pollens were found to be the main attractants and rewards to insects, and the petals played a minor role. Three species of Hymenoptera, Andrena parvula, Apis mellifera and Sphecodes pieli were the main effective pollinators. Beetles (Coleoptera), especially the Hemipyxis plagioderoides and Alleculidae sp. were also important pollinators. Syrphidae (Diptera) contributed the least to pollination.
Alcimandra cathcartii has a mixed-mating system of simultaneous self- and cross-mating by insects and wind-pollination. The fruit set, follicle set per fruit and seed set per fruit from natural pollination are significantly lower than those from either self- or cross-pollination test. The long-time stigma acceptability and the floral characteristic — the gynoecia exceed stamens found in the present study may help receive more self- or cross-pollens, however, are limited by lack of pollen vectors. The low values from natural pollination may be explained by both geitonogamy and pollen shortage.
3. Genetic diversity and the cause of formation
Our results suggest for the first time that levels of within-population genetic diversity in A. cathcartii are conspicuously low when compared with other seed plants with similar life history characteristics. If we focus on the genetic diversity index He, the total mean of within-population AFLP diversity in A. cathcartii (He = 0.1220) is nearly equivalent to that of selfers (He = 0.12) which possess the lowest genetic variation. The level of among-population differentiation (Gst = 0.2469) was also higher than the corresponding value of other species with similar life history characteristics. As a whole, the genetic variability measures showed relatively high genetic diversity in the southeast populations than others. All three cultivated populations of JZ, XC and BZ (He = 0.1153, 0.0942 and 0.0993) did not reach the genetic diversity level of their source populations: JP, BS and BS.
The Mantel test revealed a significantly positive correlation between genetic and geographic distances among populations (r = 0.6769, P = 0.9980). The UPGMA dendrogram grouped all natural populations of A. cathcartii in one cluster roughly corresponding to their geographic origin. It revealed two large groups: the southeast populations (JP, PB, WS and GN) and the western populations (JH, DU, JD, BS and YD). The Nm value of 0.7626 is lower than the criterion value (Nm ≈ 1) needed to conquer genetic drift. All these data indicate a relatively restricted gene flow among natural populations and the gene exchange is largely restricted to the nearest neighboring populations.
Based on a synthetical analysis, the observed low level of genetic diversity and high level of genetic differentiation of A. cathcartii may be primarily due to historical habitat fragmentation resulted from geologic and subsequent climatic changes or human forest-clearing activities, loss of alleles to certain extent, restricted gene flow (seed/pollen dispersal) and the breeding system with self-compatibility without sufficient cross-pollinating pollen supply.
At present, Alcimandra cathcartii is harbored in a main center of biodiversity — the southeast Yunnan Province. The comparatively high levels of genetic diversity exhibited in the southeast populations together with the high numbers of “private” AFLP fragments indicate long-term isolation of these populations in this region, rather than the involvement of recent founder events. During the course of the Himalayan Mountain building, the southeast part of Yunnan Province was isolated from the rest as discussed above; and the climate there experienced a gradual change and was thus relatively steady without glacier erosion in the Quaternary. The complex topography in this region may have helped preserve a natural refugium of biodiversity for A. cathcartii.
4. Conservation status and suggestions
Alcimandra cathcartii distributes sporadically with fragmented habitat. The species has limited ability in pollen/seed spreading and seedling renewal under natural conditions. It also exhibits a low genetic diversity and a high genetic differentiation. Although most extant populations have been brought into nature reserves, not all the in situ or ex situ populations contain the representative genetic diversity of the species, even in the southeast populations. Therefore, the extant in situ populations should be fully conserved to prevent further habitat destruction. In addition, rare alleles may be important for adapting to unusual environmental conditions. Thus, the southeast populations with the highest genetic diversity and the highest numbers of “private” fragments deserve special attention in the conservation efforts. As for the ex situ conservation plan, at least three natural populations would be required to sample more than 95% of the genetic variation in A. cathcartii. It may be worthwhile to select the genetically most divergent PB, DU or JH population which possesses more specific, locally adapted genotypes as preferential source populations in ex situ conservation programs.|