361
13.2.3.3 Natural Hybridization Between C. balansae and C. saltensis
in the Yungas
The existence of samples possessing ITS paralogs with dual origins provides a
strong evidence for the occurrence of natural hybridization between C. balansae
and C. saltensis in the Yungas. For hybridization to occur among closely related
species, several events should take place simultaneously. The first of such requirements is the overlapping distribution range of the target species (Rajora and
Mosseler 2001; Lepais et al. 2009); shared pollinators and synchronicity of flowering are also essential factors (Carney et al. 2000).
In the Yungas of NWA, the distribution range of C. angustifolia partly overlaps
with the range of C. saltensis at lower elevation of the MR, while there is no overlap
of ranges of C. angustifolia and C. balansae. The two genetic clusters of C. angustifolia derived from Bayesian analysis did not occur in any sample of C. saltensis.
Therefore, range overlapping does not seem to be enough to trigger hybridization
between C. angustifolia and C. saltensis or between C. angustifolia and putative
C. saltensis-C. balansae hybrids. According to phylogenetic studies, the clade that
contains C. angustifolia is sister to the clade that contains C. saltensis and C. balansae (Muellner et al. 2009; Koecke et al. 2013). This phylogenetic distance may
account for the absence of hybridization involving either C. angustifolia and C. saltensis or C. angustifolia and C. balansae. Studies on cross-species transferability of
SSR markers provide further insights about the phylogenetic relationships among
species of Cedrela in the Yungas (Soldati et al. 2014a).
The overlapping distribution ranges of C. saltensis and C. balansae follow a
downstream transect in the same altitudinal stratum. Studies of reproductive biology revealed that species of Cedrela may share pollinators (Bawa et al. 1985;
Kageyama et al. 2004; Ward et al. 2005; Aschero 2006; Pennington and Muellner
2010). In the Yungas, the three species of Cedrela exhibit synchronicity of the flowering period (Zapater et al. 2004). If C. saltensis and C. balansae share pollinators,
as postulated previously, and there is some degree of interfertility between these two
species, hybridization is likely to take place in sites where they co-occur.
The detection of interspecific hybrids by means of morphological characters
generally assumes that trees of hybrid origin will be phenotypically intermediate
between parental species. However, this assumption is often not valid (Allendorf
et al. 2001). For Cedrela, morphometric characters did not discriminate between
introgressant hybrids and trees harboring the expected morphology of C. balansae.
When C. balansae was studied regardless of the other two species, as was described
in this chapter and reported by Soldati et al. (2013), all samples were considered as
belonging to C. balansae. Only a joint assessment by molecular genetics of samples, a priori considered either C. balansae or C. saltensis, allowed us to detect
introgressant hybrids between them from those samples belonging to pure species.
These findings suggest that an extensive range of phenotypic plasticity exists and
that it may obscure morphological differences between C. balansae and C. saltensis. The highest levels of genetic diversity found in Calilegua National Park,
Acambuco, and San Andrés populations a priori considered as C. balansae
13 Patterns of Neutral Genetic Variation for High-Value Cedar Species…
13.2.3.3 Natural Hybridization Between C. balansae and C. saltensis
in the Yungas
The existence of samples possessing ITS paralogs with dual origins provides a
strong evidence for the occurrence of natural hybridization between C. balansae
and C. saltensis in the Yungas. For hybridization to occur among closely related
species, several events should take place simultaneously. The first of such requirements is the overlapping distribution range of the target species (Rajora and
Mosseler 2001; Lepais et al. 2009); shared pollinators and synchronicity of flowering are also essential factors (Carney et al. 2000).
In the Yungas of NWA, the distribution range of C. angustifolia partly overlaps
with the range of C. saltensis at lower elevation of the MR, while there is no overlap
of ranges of C. angustifolia and C. balansae. The two genetic clusters of C. angustifolia derived from Bayesian analysis did not occur in any sample of C. saltensis.
Therefore, range overlapping does not seem to be enough to trigger hybridization
between C. angustifolia and C. saltensis or between C. angustifolia and putative
C. saltensis-C. balansae hybrids. According to phylogenetic studies, the clade that
contains C. angustifolia is sister to the clade that contains C. saltensis and C. balansae (Muellner et al. 2009; Koecke et al. 2013). This phylogenetic distance may
account for the absence of hybridization involving either C. angustifolia and C. saltensis or C. angustifolia and C. balansae. Studies on cross-species transferability of
SSR markers provide further insights about the phylogenetic relationships among
species of Cedrela in the Yungas (Soldati et al. 2014a).
The overlapping distribution ranges of C. saltensis and C. balansae follow a
downstream transect in the same altitudinal stratum. Studies of reproductive biology revealed that species of Cedrela may share pollinators (Bawa et al. 1985;
Kageyama et al. 2004; Ward et al. 2005; Aschero 2006; Pennington and Muellner
2010). In the Yungas, the three species of Cedrela exhibit synchronicity of the flowering period (Zapater et al. 2004). If C. saltensis and C. balansae share pollinators,
as postulated previously, and there is some degree of interfertility between these two
species, hybridization is likely to take place in sites where they co-occur.
The detection of interspecific hybrids by means of morphological characters
generally assumes that trees of hybrid origin will be phenotypically intermediate
between parental species. However, this assumption is often not valid (Allendorf
et al. 2001). For Cedrela, morphometric characters did not discriminate between
introgressant hybrids and trees harboring the expected morphology of C. balansae.
When C. balansae was studied regardless of the other two species, as was described
in this chapter and reported by Soldati et al. (2013), all samples were considered as
belonging to C. balansae. Only a joint assessment by molecular genetics of samples, a priori considered either C. balansae or C. saltensis, allowed us to detect
introgressant hybrids between them from those samples belonging to pure species.
These findings suggest that an extensive range of phenotypic plasticity exists and
that it may obscure morphological differences between C. balansae and C. saltensis. The highest levels of genetic diversity found in Calilegua National Park,
Acambuco, and San Andrés populations a priori considered as C. balansae
13 Patterns of Neutral Genetic Variation for High-Value Cedar Species…