THESIS ABSTRACT
Habitat fragmentation and degradation is an important threat to biodiversity
worldwide. Heterostylous self-incompatible plants, such as Primula vulgaris, are
expected to be particularly susceptible to consequences of habitat fragmentation,
because they are putative obligate outcrossers and need cross-pollination for
successful seed production. For P. vulgaris in Flanders, habitat fragmentation has led
to small and isolated populations in degraded habitat conditions. These populations
are mainly characterized by decreased seedling recruitment, higher proportions of
older individuals and skewed morph ratios, yet still exhibit high levels of observed
heterozygosity which may represent historical variation, since an individual plant may
live for several decades. This research project examined (1) the genetic patterns of an
older and recent generation, (2) the interaction of the self-incompatibility system with
habitat fragmentation and the effects on the population genetic level, and (3) the
possibilities of existing Linear Landscape Elements (LLEs) as functional biological
corridors for pollen flow.
When we compared an old (adult) and young (recently established seedlings)
generation separately for genetic diversity and structure, the recent generation showed
significantly lower observed heterozygosities (HO), higher inbreeding coefficients
(FIS) and higher differentiation (FST) between populations compared to the adult
generation. This might result from higher levels of genetic drift and inbreeding in the
recent generation due to habitat fragmentation which might not have been as severe
for the adult generations. Other explanations are: (1) a higher survival of
heterozygotes over time because of a higher fitness of heterozygotes (heterosis) and/or
a lower fitness of homozygotes (inbreeding depression), (2) overlapping generations
in the adult life stage and (3) a lack of establishment of new (inbred) adults from
seedlings due to degraded habitat conditions.
Partial self-compatibility may be a response to ensure reproductive success when
compatible mates are scarce due to habitat fragmentation. A controlled handpollination experiment using three pollination treatments (legitimate, illegitimate and
selfing) showed partial self-compatibility for the pin morph, with very few signs of
inbreeding depression in the early development of illegitimate and selfed offspring.
This may constitute a reproductive advantage for the pin morph and may help to
maintain seedling recruitment. Investigating morph-specific genetic variation and
fine-scale spatial genetic structure in relation to morph frequency and population
demographic traits revealed no difference in the levels of genetic variation between
the pin and thrum morphs. Genetic differentiation between morph types within
populations represented only 1.8% of the total variation. Genetic diversity was
positively related to pin frequency. For the pin morph, higher inbreeding levels (FIS)
were found at high pin frequency. This may be expected given the possibility of intra
pin crossings and pin selfing with a high reproductive success and no signs of
inbreeding depression, and may partly explain higher levels of inbreeding found in the
recent generation.
The consequences of habitat fragmentation may be counteracted by the restoration of
gene flow between populations. We examined the possibilities of existing LLEs as
functional biological corridors for pollen flow using fluorescent dye particles as
pollen analogues. Dye dispersal showed a leptokurtic decay distribution at the
landscape scale, with the majority of dye particles deposited over a short-distance
range on the recipient individuals that were first visited after the source plants and
only a few long-distance transfers. The presence of a connecting LLE facilitates
pollen dispersal between populations as dye dispersal was significantly higher
between populations connected by an LLE, compared to unconnected populations.
We found evidence that pollinators use the LLEs not only as conduits between plant
populations, but also as foraging habitat. For connected populations, more dye
transfers were found when the surrounding matrix of the recipient population and of
the connecting LLE was pasture, compared to arable fields (or a mix of both). These
findings might be related to better nesting and foraging opportunities for pollinators in
pasture. The LLEs in the intensively-used agricultural landscape show a clear
potential as functional biological corridors for pollen flow. The adaptation or
restoration of LLEs between P. vulgaris populations in Flanders can certainly
contribute to ‘genetic rescue’ of the populations, by enhancing interpopulational
pollen dispersal. However, combining restoration of both habitat quality and gene
flow between populations may be indispensable to ensure a sustainable conservation
of fragmented populations.