Download chap-2 - Shodhganga

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