The population structure of any species is determined mainly by the breeding system of the species, the mechanism of pollen, fruit and seed dispersal and presence or absence of isolation mechanisms. In Piper, male, female and hermaphrodite forms exist. The cultivated P. nigrum is monoecious having hermaphrodite flowers, while the wild ones are mostly dioecious. Human selection might have played a major role in the directional evolution of hermaphroditism in the cultivated pepper. Pepper is predominantly self pollinated, and the pollen dispersal is aided by rain or dew drops, and also by the gravitational descending of pollen (geitonogamy). The flowers are protogynous, but in the absence of an active pollen transfer mechanism, protogyny becomes ineffective in ensuring outbreeding. Active and efficient pollen and seed dispersal mechanisms ensure gene flow within and between populational segments leading to the establishment of intergrading populations. The absence of any such mechanism in Piper thereby establishes effective isolation barriers among individuals and among population units. Within such units, variations can occur through segregation in the seedling progenies, accumulation of mutations and chance crossing followed by segregation. Any such variation arising in a population gets immediately fixed as a result of the prevalence of vegetative mode of propagation and such a unit may gradually diverge from other similar units. In P. nigrum dioecy occasionally can break down leading to the production of hermaphrodite flowers. Such a hermaphrodite plant climbing on a tree gradually spreads out by means of runners that climb on nearby trees, become separated from the mother vines in due course apparently because the runners get covered up by humus and soil, gradually leading to their degeneration. Simultaneously, the seeds germinate around the mother vine which also grow and climb up surrounding trees.
Thus gradually from a single vine a small population develops, consisting of the mother vine, its clonally developed progenies, sexual progenies (male, female and bisexual), the second generation progenies of the above vines, their clonal progenies and so on (Ravindran et al. 1990).
Sometimes more than one type of pepper vines (different types of P. nigrum or different species) climb up a single tree thereby increasing the chances of out crossing, resulting in hybrid seedlings. The progenies from such chance crosses grow, later climbing up the same or nearby trees and chance out crossing with parental vine or its clonal or other seedling progenies resulting in further back cross or hybrid progenies, leading to considerable variability within the population. These forces acting together in due course might have contributed to the evolution of many present day cultivars (Ravindran et al. 1990).
When male or female vines alone are present initially, the population developing out of it can be either a population of the vines of the same sex (clonal population of male or female vines), or a population of both sexes in the case of female vines. This population consists of the mother vine; its clonally propagated female vines, its seedling progenies consisting of both male and female vines, their subsequent progenies including intercrossed and backcrossed progenies (Ravindran et al. 1990).
Because of the absence of free gene flow, such populations will remain discrete and isolated from similar populations in neighbourhoods. Variations in such populations occur mainly by: (i) recombination and segregation (ii) chance crossing followed by segregation (iii) variation due to chance mutations, which will remain fixed as a result of the vegetative reproduction and (iv) isolation of discrete populational segments and the subsequent divergence of such units (Ravindran et al. 1990).
Quite often good fruit setting was noted in many isolated female plants of P. attenuatum, P. argyrophyllum, P. hymenophyllum, P. nigrum, etc. The absence of any pollen parent in the vicinity of such vines makes one to suspect apomixis as the probable cause of such high seed setting. This needs further investigation.
Biosystematical studies were carried out in Piper spp. by Rahiman and Subbaiah (1984) who made a preliminary comparative flavonoid analysis of eight species of Piper, while Rahiman and Bhagavan (1985) reported a preliminary biometrical study using D2 Statistics. Ravindran (1991), Ravindran et al. (1992) and Ravindran and Nirmal Babu (1994b, 1996) carried out detailed biosystematic studies on Piper spp. closely related to cultivated pepper, using numerical and chemical methods. They used 17 Operational Taxonomic Units (OTUs) comprising 10 species and seven accessions of P. nigrum and 30 characters to carry out Cluster Analysis and Principal Component Analysis for establishing similarity/dissimilarity among the taxa. Two cluster analysis techniques were used viz. the Average Linkage Analysis or the Unweighted Pair Group Method using Arithmetic Averages (UPGMA) for clustering the characters and the Centroid Linkage Analysis or the Unweighted Pair Group Centroid Method (UPGCM) for clustering the taxa. They reported highly significant correlations among certain characters, and the analysis led to six character clusters. They are:
1. Leaf length, leaf breadth, leaf size index
2. Fruit taste, presence of gall forming thrips
3. Leaf length/leaf breadth index, number of ribs on the leaf, ecological distribution, growth habit
4. Spike length, peduncle length, spike orientation, fruit shape
5. Leaf length/spike length ratio and spike shape
6. Leaf shape, leaf base.
The characters within each cluster are highly correlated (Ravindran 1991, Ravindran et al. 1992a). The clustering of taxa carried out by centroid linkage method led to six clusters. They are:
Cluster A — P. attenuatum, P. argyrophyllum
B - P. schmidtii, P. galeatum, P. trichostachyon C - P. nigrum, P. wightii D - P. hymenophyllum E - P. silentvalleyensis, P. mullesua F - P. longum
The first cluster consisted of P. attenuatum and P. argyrophyllum. Hooker (1886) included them under the section Eupiper. In a D2 analysis by Rahiman and Bhagavan (1985) employing five characters these two species were found to cluster with P hookeri (Syn. P. hymenophyllum). But in cluster analysis P. hymenophyllum forms an independent cluster. P. galeatum, P. trichostachyon and P. schimdtii clustered together. The first two are closely related and treated accordingly by Hooker (1886) and Gamble (1925). P. schmidtii, though a distinct high elevation species, shares certain morphological characters with the other two. Hooker (1886) included these three species in two sections the first two in section Muldera and the third one (P. schmidtii) in section Pseudochavica. The major character differentiating P. schmidtii from the other two is the nature of bracts, but also has thicker leaves and occupy a higher altitudinal niche.
All P. nigrum collections including P. nigrum var. hirtellosum were in one cluster together with P. wightii. Hooker (1886) included P. nigrum in the sect. Eupiper along with P. attenuatum, P. argyrophyllum, P. hymenophyllum, and P. wightii. But P. nigrum is quite distinct from all these species except P. wightii both morphologically and chemically. In other words the inclusion of P. nigrum with other species in one section is not supported by the cluster analysis. P. hymenophyllum forms the fourth cluster, which is distinct due to pubescence present throughout the plant body. P. silentvalleyensis and P. mullesua formed one cluster. The former is unique in having erect, flexuous, filiform spike and bisexual flower. P. mullesua is included in the section Chavica by Hooker (1886) and is the only species in the region having globose spikes. These two species resemble very much in most vegetative characters, and indistinguishable in the absence of spikes. P. longum forms the last cluster and is very distinct from all other species in having creeping habit (all others being climbers), erect cylindrical spikes and laterally fused flowers. This species is also distinct in its anatomical characters (Murty 1959). P. hapnium (though not included in the above study) is closely related to P. longum but differs from it in the climbing habit of the former.
Ravindran and Nirmal Babu (1996) using the principal component analysis identified seven Principal Components (PC) that accounted for almost all the variance observed among the OTUs. The first PC consists of leaf and fruit characters (leaf length, leaf breadth, leaf size index, petiole length, distance from the base to the second pair of ribs, plant type, fruit colour, fruit taste and thrips infestation). The second PC consists of spike length, peduncle length, spike orientation and fruit shape. The third PC consists of leaf length/leaf breadth index, rib number, growth habit and distribution. The fourth PC consists of bract type. The fifth PC consists of leaf length/spike length index and spike shape. The sixth PC consists of guard cell length, guard cell breadth, and leaf texture. The seventh PC consists of spike texture.
Distribution of OTUs between the PCs can give insight into the nature of divergence among species. P. attenuatum and P. argyrophyllum get differentiated from others mainly by PCs 2 and 4, while PC 6 differentiates P. argyrophyllum from others. P. galeatum gets separated from other taxa by PC 4. P. hymenophyllum gets differentiated by virtue of PCs 1 and 6. P. longum is distinct because of PCs 1 and 3. P. mullesua is distinct from all other species due to PCs 2, 3 and 5. PCs 1, 2, 3, 4, 6 and 7 are important in differentiating P. schmidtii, PC 4 being the most important. PCs 2, 3 and 5 are important in separating P. silentvalleyensis from all the other taxa. P. trichostachyon gets separated from other taxa by virtue of PCs 4 and 7. PC 1 is important in separating P. nigrum from other taxa, while PC 7 separates P. nigrum var. hirtellosum from P. nigrum itself (Ravindran 1991, Ravindran and Nirmal Babu 1996).
A comparative study using flavonoid profile by Ravindran (1991) and Ravindran and Nirmal Babu (1994 b) indicated the following chemical affinities:
P. galeatum - P. trichostachyon 87 per cent
P. attenuatum - P. argyrophyllum 79 per cent
P. argyrophyllum - P. hymenophyllum 78 per cent
P. galeatum - P. sugandhi 82 per cent
P. longum - P. mullesua 69 per cent
P. longum - P. silentvalleyensis 35 per cent
These chemical relationships by and large supports the taxonomical relationships arrived at by conventional tools. The members of the two sub genera are chemically very distinct, thereby lending support to the validity of the subgeneric classification (Ravindran and Nirmal Babu 1996).
Isozyme studies (Sebastian et al. 1996) showed three groups of closely related species. The first group included P. nigrum, P. pseudonigrum, P. bababudani, and P. galeatum. The second group consisted of P. chaba, P. hapnium and P. colubrinum. P. argyrophyllum and P. attenuatum formed the third group. P. longum and P. betle were distinct from all others. The reliability of isoenzyme studies in elucidating taxonomic relationships is doubtful, because a very distant species such as P. colubrinum is grouped with two other species which are morphologically and cytologically quite distinct. Moreover Sebastian and Sujatha (1996) found that the isozyme patterns of peroxidase and esterase are different in different plant parts and vary according to the developmental stage.
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