Reciprocal Translocation in Chestnut (Castanea spp)

The American chestnut (Castanea dentata, 2n = 2x = 24), once known as the King of the Appalachian Forest, has been decimated by chestnut blight caused by an invasive fungus (Cryphonectria parasitica).  Chinese chestnut (C. mollissima) is resistant to the fungus and the trait is being transferred into American chestnut through backcross breeding by The American Chestnut Foundation (TACF).  Interspecies crosses often unveil structural differences such as translocations, inversions, and/or insertions/deletions of chromosomal segments.  Kubisiak et al. (1997) first developed a genetic map of chestnut using an inter-specific F2 family where the formation of two of the 12 genetic linkage groups were not resolved but formed a single complex group.  A later discussion of these results raised a possibility of the presence of a reciprocal translocation (RT) involving linkage groups (LGs) B and E.  In addition, our unpublished data of a meiocyte analysis of an F1 tree supported the reciprocal translocation hypothesis.   More recent genetic mapping data (unpublished) has indicated that a C. mollissima genotype is heterozygous for a translocation involving LG_H and LG_L.  We have used BAC-FISH (fluorescent in situ hybridization) to explore this apparent H/L translocation in pachytene spreads of the indicated genotype.  Four of the six BACs (all labeled with digoxigenin hapten and detected with green fluorochrome), selected across the entire length (2.7, 16.0, 22.5, 31.0, 35.6 and 47.0 cM) of LG_L, hybridized to about 40% of the pachytene chromosome and the other two BACs hybridized towards the distal 10% of the opposite arm of the same chromosome, leaving approximately about 50% of the chromosomes with no BAC-FISH signal.  In addition, one of seven BACs (all labeled with biotin hapten and detected with spectrum orange fluorochrome), selected across the entire length (1.3, 6.3, 14.9, 17.5, 36.0, 46.0 and 57.9 cM) of LG_H hybridized close to one of the LG_L BACs (positioned third from one end of the chromosome or positioned fourth from the other end) and leaving about 50% of the chromosome with no BAC-FISH signal.  This undetected portion of the LG_L chromosome could be a region of a translocated segment from the LG_H chromosome.  Additional research is needed to confirm this hypothesis of Reciprocal Translocation in Chestnut.