Genetics 15: 844-845 (1929)
CHROMOSOME BEHAVIOR IN SORBOPYRUS AND SORBARONIA
KARL SAX
ARNOLD ARBORETUM LABORATORY, HARVARD UNIVERSITY
Communicated October 9, 1929

In the Pomoideae subfamily of the family Rosaceae there are a number of interesting natural genus hybrids. Among these hybrids recognized by Rehder2 are Sorbaronia (Sorbus X Aronia), Sorbopyrus (Pyrus X Sorbus) and Amelasorbus (Amelanchier X Sorbus). The relative frequence of such hybrids suggests that many of the genera in this subfamily may be rather closely related and that generic hybrids of horticultural value might be produced.

All three of the above generic hybrids are growing in the Arnold Arboretum but the plants of Amelasorbus are not large enough to produce flowers. The other two hybrids bloom freely and produce fruits each year. Sorbaronia Dipelii (Sorbus Aria X Aronia melanocarpa) is completely fertile and sets seeds abundantly. Sorbopyrus auricularis bulbiformis (Pyrus communis X Sorbus Aria) produces fruit but few seeds are developed. This year only five good seeds were obtained from more than fifty fruits.

Cytological studies on these hybrids were made to determine the chromosome compatibility and behavior. Permanent smears of the pollen mother cells were made which were fixed in Nawaschin's solution and stained with crystal violet iodine.

In Sorbaronia Dippelii p.m.c. there are 17 pairs of chromosomes at the first reduction division. Both reduction divisions are perfectly normal and all of the pollen grains appear to be functional. There are 17 pairs of chromosomes in Sorbus Aria. Counts have not been obtained for Aronia melanocarpa but in view of the chromosome number and behavior in the hybrid both parents evidently have 17 gametic chromosomes which are completely compatible in the F1 hybrid.

Sorbopyrus auricularis bulbiformis in the Arboretum is from scions sent from the Kew Gardens in England. This variety of Sorbopyrus is a seedling of auricularis and is more like the pear in fruit characters than the parental species.3 A cytological study of the pollen mother cells shows that there are 17 paired and 17 single chromosomes at the first reduction division. Numerous counts show a total of 34 chromosomes at metaphase. In later stages of division 17 chromosomes can be counted at each pole and 17 dividing univalents between. The second division is also irregular and practically all of the pollen grains are aborted. Sorbus Aria has 17 gametic chromosomes and the species of Pyrus investigated by Dr. Nebel1 also have 17 pairs of chromosomes. This variety of Sorbopyrus is evidently a back cross of a diploid egg cell of the F1 hybrid with a haploid pollen grain from Pyrus. The presence of two sets of Pyrus chromosomes and one set of Sorbus chromosomes would explain why the variety is more like Pyrus than the parental hybrid. Evidently the Sorbus chromosomes will function in somatic development with either one or two sets of Pyrus chromosomes although the Pyrus and Sorbus chromosomes do not pair at the reduction divisions.

The chromosome behavior in these generic hybrids suggests that it may be necessary to make a taxonomic revision of the genera in the Pomoideae subfamily of the Rosaceae.

  1. Nebel, B., Zur Cytologie von Malus und Vitis. "Die Gartenbauwissenschaft," Band 1, Heft, 6, 449‑592 (1929).
  2. Rehder, A., Manual of Cultivated Trees and Shrubs, The McMillan Co., New York, 1927.
  3. Tatar, M. "Pirus Bollwylleriana De. C. var. bulbiformis," Wien. Obst‑ud Gart. Zeitung., 3, 26‑28 (1878).