Here is some of the work that I’ve been doing for my PhD. I have an interest in creating wide hybrids using various techniques, such as somatic hybridization, traditional breeding, and embryo rescue. This project is a mix of these methods.
In 2013 I started making some fusions between Citrus varieties and Citropsis gilletiana. Citropsis is a genus consisting of 12 or so species, all found in Africa, and mostly in central Africa. They are known as cherry oranges, due to the clustering of fruit similar to cherries, but they don’t have long stems like cherries. Most I believe are inedible or at least not tasty. There is one species (or subspecies) that was found and described as having delicious fruit: Citropsis gabunensis var. lacourtiana – Sankuru cherry orange. Unfortunately, it is not found in cultivation and I have been unable to locate someone in the Democratic Republic of Congo to send me some. (I had one contact that said he would, but never materialized.) Of the species described (there could be more species undescribed), only a few of them can be found in botanical collections. In the U.S. for example, there are four species found in the Citrus Variety Collection (UC Riverside): Citropsis daweana (Mozambique cherry-orange), Citropsis gabunensis (Gabon cherry-orange), Citropsis gilletiana (Gillet’s cherry-orange), and Citropsis schweinfurthii (Uganda cherry-orange). Here in Florida, the Department of Plant Industry (DPI) has one species, C. gilletiana, which originally came from the same collection as the clone in Riverside. And as I understand it, clones in other Citrus collections of the world came from the US collection, most of which were made prior to 1960. All of this means there is very little genetic diversity of Citropsis within the U.S. (http://www.citrusvariety.ucr.edu/citrus/relatives.html)
The reason I want to make crosses with Citropsis is to develop rootstocks for Citrus. C. gilletiana, specifically has shown resistance to the burrowing nematode (Radophilus similis) and Phytophthora complex of diseases. Citropsis sp. are graft compatible with Citrus, but only marginally sexually compatible. Malcolm Smith, an Australian Citrus breeder, was able to make crosses using Citrus wakonai and Citropsis gabunensis. However, the resulting hybrids have been sterile. There was another Japanese researcher looking at cross-compatibility to investigate genetic distance, and made some hybrids with Citropsis, but these were also sterile. There was a University of Florida researcher, Harry Ford, who was attempting to make Citropsis crosses in the 60’s and 70’s, but he must not have been successful because there were no further publications.
There are a couple of reasons Citropsis sp. are a poor choice for a rootstock. Citropsis sp. are tropical, and have little to no cold tolerance (necessary since Florida is subtropical, and experiences the occasional freeze), and Citropsis sp. make zygotic seeds (as opposed to nucellar seeds, which are clones of the mother). Nucellar seeds are preferred for traditional rootstock propagation; since each seed is a clone of the mother, it makes for uniform seedlings. So if a fruiting hybrid could be made, this could be useful, as there is no commercially available rootstock resistant to the burrowing nematode.
I made a poster about this process, with cited literature: