Hybridization in Strawberries

Strawberry, Fragaria virginiana glauca from Uintah Mountains, Utah (left),similar to the one used in original hybridization leading to breeding of California day-neutrals (source of genes conditioning day- neutrality). ‘Hecker’(right) is a third backcross derivative (1979 release). many important genes, including those gov- The second approach uses 9-ploid (2n = erning the ability to grow under rigorous en- 9 x = 63) plants in backcrossing to octoploid vironmental conditions. Recently, a new cultivars, relying on the high probability that group of day-neutral cultivars has been de- the 9-ploids will be very fertile, and virtually rived directly from hybrids between ‘Shasta’ all of the gametes will result from normal re- Hybridization in and an E virginiana male plant from the Wa- duction. The strategy is simply to select for strawberries satch Mountains of Utah through backcross- the target gene or combination of genes from ing. The group includes the California culti- the diploid species and then, through back- vars ‘Aptos’, ‘Brighton’, and ‘Hecker’ and crossing, incorporate those genes into the de- Royce S. Bringhurst the USDA-Maryland cultivars ‘Tribute’ and rivatives, finally, by eliminating undesirable Victor Voth ‘Tristar’, all released recently, Starting with chromosomes, arriving back at the octoploid specific target genes in wild octoploids, only level. The simplest way to generate 9-ploids three to four backcrosses are required to in large numbers has been to backcross the El yploid y is important in strawberries, be- transfer a given trait into useful cultivars. pentaploids already mentioned to the desired cause only in the end products found in na- In developing interspecific hybrids with octoploids, relying again upon the fact that ture (octoploids Fragaria chiloensis and R lower ploidy levels, genes from diploid (2n = most of the offspring of the pentaploids will virginiana) were the necessary genes found, 2x = 14), tetraploid (2n = 4x = 28), and be from unreduced gametes. organized, and conserved in such a way as to hexaploid (2n = 6 x = 42) Fragaria species All or part of the uses of diploid and tetra- make possible the relatively rapid breeding of can be transferred to the octoploid cultivars ploid species in hybridization with octoploids the modern large-fruited strawberry culti- in more indirect ways. Only transfers involv- have been carried out with diploids R vesca, vars. Lower ploidy levels are of interest, be- ing diploids and, to a lesser extent, tetraploids R viridis, E iinumae, R nubicola, and R ye- cause they tend to be highly specialized to will be discussed here. zoensis and with the tetraploid R orientalis. specific environments, and many of the traits Two approaches have been explored, and Much of the study has involved natural pen- they carry as a result may be useful in the cul- both are feasible. The first involves going to taploid and hexaploid hybrids between octo- tivars of the future, if they can be introduced the next higher chromosome level, decaploid ploid California E chiloensis and diploid Cal- into the octoploids. (2n = lox = 70) through natural doubling ifornia R vesca found along the California Only a small part of the rich germplasm of the chromosome number. This has been coast. pool available for strawberry breeding has accomplished in a number of ways, including: Intergeneric hybridization between Fra- been utilized thus far, and genetic accessibil- OHybridizing the octoploids with the di- garia and Potentilla species offers many in- ity has been the deciding factor in determin- ploids to produce pentaploids (2n = 5 x = teresting possibilities. Two fertile colchicine- ing what has been used. The most accessible 35) and doubling the chromosome number induced amphiploids have been reported, one species have been American E chiloensis and with colchicine. a decaploid hybrid between octoploid R X E virginiana, ancestors of modern cultivated 0Intercrossing or self-pollinating the pen- ananassa and P. palustris giving a 14-ploid strawberries (E X ananassa), since all are oc- taploids, relying on the higher incidence of (2n = 14 x = 98) and the other a decaploid toploids (2n = 8 x = 56 chromosomes). functional unreduced male and female ga- hybrid between octoploid E chiloensis and In interspecific hybridization at the octo- metes to yield decaploids. diploid P. glandulosa (2n = 10 x = 70). The ploid level, R chiloensis has been the source OUsing natural tetraploids or induced te- latter decaploid is of particular interest, be- of many of the genes presently exploited in all traploids (from diploids) to hybridize with cause it can be crossed with the decaploid I;: important cultivars, including those govern- the octoploids to produce hexaploids (2n = chiloensis and R X ananassa x E vesca hy- ing disease resistance and those that condi- 6 x = 42), and in turn backcrossing the hex- brids, opening the way for possible transfer tion ability to grow vigorously during the aploids to the octoploid, again relying on the of Potentilla genes to octoploid strawberries winter. The latter probably trace back to Et- high incidence of unreduced gametes in the via backcrossing with the resulting 9-ploid ter’s ‘Fendalcino’, a California R chiloensis hexaploids to yield decaploids. hybrid. derivative and ancestor of ‘Lassen’, the culti- ODeveloping 16-ploids (2n = 16~= var upon which the southern Calfornia com- 112) by colchicine treatment of octoploid ga- Royce S. Bringhurst, Professor, Pomology, U.C., Davis, and Victor Voth, Pomologist, mercial strawberry industry was built. metes in hybridization with the above-men- Davis, stationed at the South Coast Field Station, I? virginiana has also been the source of tioned tetraploids. Santa Ana. CALIFORNIA AGRICULTURE, AUGUST 1982 25 .

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