94
impact of abundance of parental species, directionality of gene flow, gamete competition and hybrid fitness in the introgression dynamics on a local scale. His model
suggests that introgression may be structured along ecological gradients due to
changes in the relevance of different reproductive barriers.
In 2017, El Mujtar et al. begun to test the model by studying the pattern of introgression in a natural hybrid zone of these species at Lake Lácar basin. This study
focused on two plots 280 m apart in altitude (ca. 1.9 °C difference in mean temperature) and two subplots which captured microsite variation (abundance and spatial
distribution of species and predominance of wind direction) within each plot. Based
on intensive sampling of individuals (2055, including adults and regeneration) and
molecular genotyping with six highly species-specific nuclear microsatellites, the
study revealed that introgressive hybridisation occurs at a global rate of 7.8% and
that different types of reproductive isolation barriers could be acting along the altitudinal gradient. At low altitude, introgression occurs at a rate of ca. 11% with a
unimodal distribution of hybrid genotypes, suggesting weak reproductive barriers.
In contrast, at high altitude, introgression occurs at a rate of ca. 6% with an asymmetric bimodal distribution, suggesting stronger reproductive barriers. Moreover,
F1 hybrids were detected at a global frequency of 3.8% and are fertile, according to
the detection of first- and late-generation hybrids. Further studies are necessary to
determine the relative contribution of pre- and post-zygotic reproductive isolation
barriers to the introgression pattern detected along the altitudinal gradient (i.e. surveys of phenological variation, genotyping of seed samples, parentage analysis,
evaluation of relative hybrid fitness).
El Mujtar et al. (2017) corroborated some predictions of Gallo’s conceptual model
(Gallo 2002) and contributed with new elements related to the direction and generation
(early and late) of introgression along the altitudinal gradient. Other predictions such as
the asymmetry of hybridisation could not be evaluated as divergent cytoplasmic markers are not available. Other hybrid zones, including altitudinal and rainfall gradients,
should be studied in order to understand N. alpina and N. obliqua introgressive hybridisation dynamics at a more generalised level and the role of this evolutionary process in
the response of Nothofagus species to climate change. Furthermore, such studies,
together with long-term field trials, might contribute to consider the potential relevance
of hybrids as valuable entities for breeding these two species.
4.2 Genetic Variation Patterns Based on Neutral
Genetic Markers
4.2.1 Inference of Past Demographic Changes: The Impact
of the Pleistocene Glaciations
Climatic oscillations during the quaternary strongly affected the distribution of
warm-temperate ecosystems, with repeated glaciations occurring since 1.8 million
years BP, from the Pleistocene to the Holocene. These processes affected the
M. M. Azpilicueta et al.
impact of abundance of parental species, directionality of gene flow, gamete competition and hybrid fitness in the introgression dynamics on a local scale. His model
suggests that introgression may be structured along ecological gradients due to
changes in the relevance of different reproductive barriers.
In 2017, El Mujtar et al. begun to test the model by studying the pattern of introgression in a natural hybrid zone of these species at Lake Lácar basin. This study
focused on two plots 280 m apart in altitude (ca. 1.9 °C difference in mean temperature) and two subplots which captured microsite variation (abundance and spatial
distribution of species and predominance of wind direction) within each plot. Based
on intensive sampling of individuals (2055, including adults and regeneration) and
molecular genotyping with six highly species-specific nuclear microsatellites, the
study revealed that introgressive hybridisation occurs at a global rate of 7.8% and
that different types of reproductive isolation barriers could be acting along the altitudinal gradient. At low altitude, introgression occurs at a rate of ca. 11% with a
unimodal distribution of hybrid genotypes, suggesting weak reproductive barriers.
In contrast, at high altitude, introgression occurs at a rate of ca. 6% with an asymmetric bimodal distribution, suggesting stronger reproductive barriers. Moreover,
F1 hybrids were detected at a global frequency of 3.8% and are fertile, according to
the detection of first- and late-generation hybrids. Further studies are necessary to
determine the relative contribution of pre- and post-zygotic reproductive isolation
barriers to the introgression pattern detected along the altitudinal gradient (i.e. surveys of phenological variation, genotyping of seed samples, parentage analysis,
evaluation of relative hybrid fitness).
El Mujtar et al. (2017) corroborated some predictions of Gallo’s conceptual model
(Gallo 2002) and contributed with new elements related to the direction and generation
(early and late) of introgression along the altitudinal gradient. Other predictions such as
the asymmetry of hybridisation could not be evaluated as divergent cytoplasmic markers are not available. Other hybrid zones, including altitudinal and rainfall gradients,
should be studied in order to understand N. alpina and N. obliqua introgressive hybridisation dynamics at a more generalised level and the role of this evolutionary process in
the response of Nothofagus species to climate change. Furthermore, such studies,
together with long-term field trials, might contribute to consider the potential relevance
of hybrids as valuable entities for breeding these two species.
4.2 Genetic Variation Patterns Based on Neutral
Genetic Markers
4.2.1 Inference of Past Demographic Changes: The Impact
of the Pleistocene Glaciations
Climatic oscillations during the quaternary strongly affected the distribution of
warm-temperate ecosystems, with repeated glaciations occurring since 1.8 million
years BP, from the Pleistocene to the Holocene. These processes affected the
M. M. Azpilicueta et al.