First generation hybrids between this tetraploid and diploid species are very rare, probably because the odd number of chromosomes (3n, triploid) in the fertilized ova can’t pair up during cell division. Out of more than two hundred controlled pollinations, I have obtained fewer than 10 viable seedlings. All of these F1 (primary, first generation) crosses are tetraploid and strongly resemble the “arabicum” parent. It’s apparent that “arabicum” occasionally makes triploid ova, which, when fertilized by a haploid “obesum” sperm cell, forms two pairs of each homologous chromosome (Richard J. “Jake” Henny, written communication). Therefore F1 hybrids are 3/4 "arabicum", which explains the dominance of this species' characteristics in the hybrids.

 

Fortunately, some of the hybrids are fertile. The F1 and subsequent generations of this breeding line have impressive hybrid vigor, and can rapidly grow into huge specimens. The first one, named ‘Arabian Ruby’ is the most vigorous adenium that I have ever seen. At ten years of age, it filled a 42-inch (107 cm) pot with a multi-stemmed base almost three feet (91 cm) thick. Cuttings of this plant are also vigorous and very caudiciform. Most other plants of this interspecific hybrid are also very large.

 

In subsequent generations the “arabicum” parentage remains heavily dominant in plant form. The “obesum” parentage shows in the large, brightly colored flowers (three to four inches, 76-102 mm wide), and unfortunately, a tendency to weak stems. After four generations of selection, some excellent reds are showing up. See the superior cultivars page for a gallery of these hybrids.