“Dr. Faridi is one of a very few scientists worldwide who can do this work,” noted Dana Nelson, project leader for the Southern Institute of Forest Genetics that manages the cytogenetics lab. “Faridi can literally watch through a microscope as a chestnut pollen cell goes through meiosis, the elegant process by which the paired chromosomes from two parents recombine into a single set of chromosomes ready to fertilize an egg cell.”

Mismatched Pairs

Chinese and American chestnut trees both have 12 pairs of chromosomes, but Faridi found that their chromosomes differ structurally from one another, which can create problems when the two trees are crossed. Faridi started looking at pairing between Chinese and American chestnut chromosomes during meiosis in first-generation hybrids, which were half Chinese and half American chestnut. What he’s seen in hybrid chestnut mother pollen cells isn’t exactly good news.

“One problem is that one of the chromosome pairs from the two species doesn’t really match up,” says Sisco. “So when it comes time for the Chinese and American chestnut chromosomes to exchange genes during recombination, they don’t line up precisely. They pair only on the ends, leaving a bulge in the middle. In genetics, we call this an inversion.”

Another problem involves 2 other pairs of the 12 chromosomes. Eons ago chromosomes from one pair broke and recombined with another pair to cause what is called a translocation. In the Chinese/American chestnut hybrid, four chromosomes come together into a cross shape instead of the normal linear shape. Translocation, like inversion, hinders the exchange of genes between the two species and causes some pollen to abort.

And what if the genes for resistance are on those areas that don’t match up? “There’s some evidence that this might be the case for at least two of the three genes involved in resistance that have been identified so far,” says Sisco. “The chromosomal differences between Chinese and American chestnut can either work for or against us in transferring Chinese resistance to American. The hope is that the genes for resistance are out on the ends of the chromosomes, where the strands meet and interlock so they can be easily transferred from Chinese to American chestnut without bringing along a large block of Chinese genes.”

If a resistance gene is on one of the sections that doesn’t interlock, a large segment of the Chinese genetic code might get carried over along with the gene when the chromosomes combine, which would make it more difficult to breed an almost pure, but resistant American chestnut. Faridi’s next project, partly funded by a TACF grant, is to see whether known resistance genes are on the chromosomes that have abnormal pairing, and, if they are, whether the genes are in the middle of the chromosomes, where pairing is hindered, or at the ends, where pairing is more normal. —ZH

For more information:
Nurul Faridi at 979–862–3908 or nfaridi@tamu.edu
Dana Nelson at 228–832–2747 or dananelson@fs.fed.us