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II. Review of literature: The literature relevant to the work carried out in the present investigation has been reviewed in the following pages under 4 heads: II.1 Conservation of threatened plant species: Conservation of threatened plant species is being pursued all over the world and has become a popular field of research after the development of conservation biology as a distinct discipline during 1980s. Recognition of the importance of biodiversity and the threat posed to mankind due to its depletion was made as early as late 19th century and its conservation was strongly advocated (Cloyd, 1972; Evans and David, 1997). At that time human intervention, overexploitation or natural catastrophes were regarded as main reasons for depletion of plant as well as animal species and conservation efforts were mostly concentrated around providing protection to the natural habitats of these species and imposing restrictions on undue human intervention into these areas (Carr et al., 1994). The conservation of natural habitats of threatened species i.e. in situ conservation is being followed even at present and is also regarded as the best approach to conservation as it allows the species to grow, reproduce and evolve in their natural habitats (Primack, 1993; Frankel et al., 1995; Kushalappa et al., 2001). But it was observed that many of the endangered species have not recovered even after protection and restoration of their habitats (Bevill et al., 1999). In such cases, ex situ conservation is the only choice left over (Withers, 1979; Conway, 1980; Dresser, 1988; Seal,1988; Cohn, 1991; Frankel et al., 1995; Chandel et al., 1996; Verma, 2001).With the emergence of conservation biology as an integrated approach to understand the working of nature, ecosystems and species, inputs from diverse fields of study have contributed a lot in understanding the actual cause of threat to the 10 species and thus managing and conserving them according to their specific requirements. It was observed that even within protected areas, many intrinsic factors limit the abundance and survival of species. II.2 Species extinction - reasons and consequences: Anthropogenic stress is considered as a major reason of species extinction (Bevill et al., 1999; Primack, 1993), acting mostly through habitat destruction (Le Houerou and Gillet, 1986; Murphy and Lugo, 1986; Myers, 1986; Saunders et al., 1987; Ehrlich, 1988; Janzen, 1988; Williams, 1990; Mitchell, 1990; Moyle and Leidy, 1992; Breman, 1992; Luijten et al., 1996). Change in land use patterns also aggravate magnitude of threat to plant species (Qureshi and Kaul, 1970; Nayar and Sastry, 1987; Critchley, 2000). Besides human intervention, several other factors contibute to species erosion. Natural catastrophes, environmental and genetic stochasticity, invasion of exotic species, over harvesting, forest fires, acidification of atmosphere and overharvesting are among some important drivers of species erosion (Lucas and Synge, 1978; Mennema et al., 1985; Nayar and Sastry, 1987; Fennema, 1990; Dueck and Elderson, 1992; Murcia, 1995; Luitzen et al., 1996; Koul et al., 1997, 2000; Evans et a., 2000; Stohlgren et al., 2000; Bruna et al., 2001; Cunningham and Duncan, 2001; Prasad and Purohit, 2001; Sudha and Seeni, 2001). It was pointed out by various researchers that extinction of species is occurring at an alarming rate, 10 - 10,000 times faster than the natural rate of extinction ( Jablonski, 1991; May et al., 1995; Primm et al., 1995). It has been reported that India is losing 47,500 hectares of its forest cover each year and it can lead to a loss of 10% of its endemic and endangered plant species (Nayar, 1995, 1997). 11 II. 3 Reproductive biology of threatened plant species: a tool for their conservation: Two important areas of research in the field of conservation biology of threatened plant species are the population genetics and reproductive biology. Reproduction is the most fundamental requirement for a species to grow and evolve and thus, understanding the process of reproduction right from gametogenesis to seed germination is crucial in finding out the constraints that limit the perpetuation of species in nature (Verma et al., 2001, 2004, 2008). Several aspects of reproductive biology i.e. flowering phenology, self and cross compatibility, pollinator availability, success of pollination, reproductive success, seed and fruit formation, seed germination and seedling recruitment are crucial in understanding the reproductive ecology of plant species (Ollerton and Lack, 1992). Reproductive failures are often encountered in rare plant species having small populations. The small populations in the form of patches result from habitat loss and fragmentation and face an increased risk of extinction (Saunders et al., 1991; Schemske et al., 1994). In species with highly reduced distribution, there occurs depletion in the level of heterozygosity and genetic diversity due to increased inbreeding and as a consequence such populations show a reduction in fitness and adaptability to environmental changes. Population is the evolutionary unit of a species (Davis and Heywood, 1973) and according to the concept of minimum viable population a reduction in population size beyond certain thresh hold directly effects its long term survival and evolutionary flexibility (Shaffer, 1981; Frankel, 1983; Soule, 1987; Menges, 1991). It has been observed that effective population size of less than 100 individuals makes a plant species prone to severe inbreeding depression (Frankel et al., 1995). Interpopulation gene flow through pollen 12 is severely affected if populations shrink in size and distance between different populations increases (Barrett and Kohn, 1991; Ellstrand and Elam, 1993; Oostermeijer et al., 1995; van Treuren et al 1996; Young et al., 1996; Schaal and Leverich, 1996; Fischer and Matthies, 1998; Young and Brown, 1999; Luijten et al., 2000; Fischer et al., 2003; Willi et al., 2005; Willi et al., 2006;). Population structure and reproductive biology studies in threatened species provide a direct understanding of the reproductive hiccups and constraints that need to be considered in developing conservation strategies for them. Knowledge of the genetic diversity stored in a population is immensely important in selecting the best genotypes and also the most critical populations for conservation. It has been shown that, geographical range, breeding system, longevity and mode of dispersal of pollen and plant propagules determine the distribution of genetic diversity within and among the populations (Hamrick and Godt, 1989, 1996a, b). Studies on various aspects of reproductive biology i.e. flowering phenology, pollination biology, breeding and meiotic systems, seed viability etc. of several threatened taxa have been made and have provided valuable information regarding the constraints in the process of sexual reproduction at various levels which contribute towards decline in population viability and threatening the survival of these species in nature. Results of such studies in a number of threatened plant species have helped in planning effective conservation and management programs (Synge, 1981; De Mauro, 1993; Weller, 1994, Evans et al., 2000; Verma et al., 2001, 2004, 2008). Reproductive constraints may arise in plants right at the time of gametogenesis because of meiotic anomalies thus disturbing the process of origin of genetic variation and its distribution through male and female gametes. A number of studies report 13 irregularities including synaptic mutations (Jauhar and Singh1969; Krishnan et al., 1970; Jackson et al., 2001; Vergilio et al., 2008; Sharma et al., 2010; Sharma et al., 2011; Pagliarini et al., 2011), abnormal spindle organization (Dawe, 1998; Koduru and Rao, 1981) and cytomixis (Lattoo et al., 2006; Song and Li, 2009) in pollen mother cell (pmc) meiosis in different taxa which affects the viability of the resultant pollen and thus affecting the reproductive success in such species adversely. Three species of genus Valeriana, V. scandens, V. nitida and V. officinalis growing in Europe suffer from pollen sterility due to disturbances during different stages of microsporogenesis. In V. scandens immature pollen degeneration and malformation of sporoderm leads to sterility (Duarte-Silva et al., 2010) where as in V. officinalis and V. nitida precocious senescence of tapetum and aneuploidy in meiosis respectively lead to sterility (Shugaeva, 1972, 1979). Besides the abnormalities in the male track, some species like Manglietia aromatica of family Magnoliaceae have become endangered due to abnormal development of female gametophyte and degeneration of egg apparatus before fertilization (Yue - Zhi et al., 2003). Pollination biology is an important parameter in the life history of flowering plants that determines the success of sexual reproduction. It is also one of the most studied aspect of reproductive biology of threatened plant species. Plant - pollinator mutualism dates back to the cretaceous period and nearly 67% of angiosperms depend upon insects for pollination (Tepedino, 1979). Numerous studies on pollen or pollinator limitation have been made and found it to be the cause of reproductive failure and endangerment of several taxa (Jennifer et al., 2001; Verma et al., 2008; Trembly et al., 2005). Pollen limitation of reproductive success is common in plants 14 growing in harsh climatic conditions like in alpine zones (Arroyo et al., 1982; Bingham and Orthner, 1998; Medan et al., 2002). Pollen and pollinator limitation in small and restricted populations further leads to the failure of sexual reproduction (Wilcock and Neiland, 2002). Mating and breeding system of some threatened taxa have been found to be detrimental in designing the genetic structure of populations which have already declined to small size. Historically large and outcrossing populations suffer due to inbreeding depression caused by forced selfing in case the mating system is self - compatible and total loss of reproductive success in case the species is self - incompatible (Schemske and Lande, 1995; Lande, 1988; Menges, 1991). In species like Linnaea borealis small population size, distribution in the form of small patches, self incompatibility and clonal propagation lead to a condition of scarcity of suitable mate and nearly no fruit or seed set in nature (Scobie and Wilcock, 2009). This is due to the prevalence of geitonogamy in the patches derived from the clonal multiplication of a single progenitor plant. Under such conditions, incompatible pollen from plants having the same S - allele gets deposited on the stigma leading to the total failure of reproduction in these populations (Charpentier, 2000; Araki et al., 2007; Wilcock and Jennings, 1999). The declining populations of an extremely rare cactus species, Opuntia spinosissima were also found to face reproductive failure due to the scarcity of pollen from the suitable mating type leading to no fruit or seed set in such populations (Ortiz, 1998). Detailed investigation of reproductive biology of Coincya rupestris subsp. rupestris, a critically endangered endemic plant of Mediterranean region revealed that despite high fecundity, the species suffers due to inter-specific competition and water availability during seedling establishment stage. 15 Role of pollinators in ensuring reproductive success in this species was also strongly emphasized. Based on these observations, necessary suggestions for the conservation of this species were made. These include prohibiting the use of insecticides and phytochemicals in agricultural areas near the habitats of this species and preventing disturbance of the natural habitats due to recreational activities and grazing (Copete et al., 2008). Several plant species are known to posses specific adaptive strategies to combat loss of pollinators and also to modify their breeding system towards a more favourable mode i.e. outcrossing. Delayed selfing through stylar movement assures reproductive success in some species (Li et al., 2001; Liu et al., 2001; Sun et al., 2007; Ruan et al., 2009) to avoid autonomous selfing and promote outcrossing (Verma et al., 2004). Flexistyly is another mechanism that promotes outcrossing in some plant species (Li et al., 2001). II.4 Knowledge of the level of genetic diversity in threatened taxa – a prerequisite to design strategies for their conservation: Determination of the extent of genetic diversity is one of the important prerequisites to evolve a systematic conservation strategy for a species. It helps in understanding the magnitude and distribution of variability within and among the populations thus helping in prioritizing sites for in situ conservation and also in developing effective sample collection strategies for ex - situ conservation (Holsinger and Gottlieb, 1991; Godt et al., 1996; Petit et al., 1998; Hogbin and Peakall, 1999; Newton et al., 1999; Neel et al., 2003). Level of genetic diversity in a species and its pattern of distribution within and among different populations is a manifestation of the breeding and mating system and pollination mechanisms operating in a species. 16 Pattern of genetic diversity in a population also determines its ability to adapt to climatic conditions and relative fitness of different genotypes. With the advent of molecular markers (both allozyme and DNA based), it has become possible to estimate and analyze the genetic variation directly without undergoing the tedious and lengthy procedures of population genetics. A number of studies on assessment of the extent of genetic variation in threatened plant species have been undertaken (Hamrick et al., 1989; Hamrick and Godt, 1996; Gitzendanner and Soltis, 2000; Neel and Ellstrand, 2003); Verma et al., 2007; Lattoo et al., 2007; Yang et al., 2010; Gupta et al., 2012; Gupta et al., 2012; Raina et al., 2012; Warghat et al., 2012). Such studies are helpful in identifying the populations most needful of conservation due to low levels of genetic variability and also in selecting elite genotypes for conservation and improvement. Besides, phylogenetic relatedness of different populations and extent of their diversification can also be predicted using such studies. 17