Bringhurst, Royce S. : Royce S. Bringhurst, Professor, Pomology, U.C., Davis

Voth, Victor

Strawberry , Fragaria virginiana glauca from Uintah Mountains , ( left ) , similar to the one used Utah in original hybridization leading to breeding of Californiaday - neutrals ( source of genes conditioning day - is a neutrality ) . â?? Hecker â?? ( right ) ( 1979 third backcross derivative 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 backcrossingto octoploid vironmental conditions . Recently , a new cultivars , relying on the high probabilitythat the 9 - ploids will be very fertile , and virtually group of day - neutral cultivars has been de - rived directly from hybrids between â?? Shasta â?? all of the gametes will result from normal re - Hybridization in and an E virginianamale plant from the Wa - duction . The strategy is simply to select for the target gene or combination of genes from satch Mountains of Utah through backcross - strawberries the diploid species and then , through back - ing . The group includes the California culti - vars â?? Aptos â?? , â?? Brighton â?? , and â?? Hecker â?? and crossing , incorporate those genes into the de - Royce S . Bringhurst rivatives , finally , by eliminating undesirable the USDA - Maryland cultivars â?? Tribute â?? and Victor Voth chromosomes , arriving back at the octoploid â?? Tristar â?? , all released recently , Starting with level . The simplest way to generate 9 - ploids specific target genes in wild octoploids , only in large numbers has been to backcross the three to four backcrosses are required to Elyploidy is important in strawberries , be - pentaploids already mentioned to the desired a given trait into useful cultivars . transfer in the end products found in na - cause only octoploids , relying again upon the fact that In developing interspecific hybrids with ture ( octoploids Fragaria chiloensis and R will most of the offspring of the pentaploids = lower ploidy levels , genes from diploid ( 2n virginiana ) were the necessary genes found , be from unreduced gametes . 2x = 14 ) , tetraploid ( 2n = 4x = 28 ) , and organized , and conserved in such a way as to All or part of the uses of diploid and tetra - hexaploid ( 2n = 6 x = 42 ) Fragaria species make possible the relativelyrapid breeding ploid species in hybridization with octoploids of can be transferred to the octoploid cultivars have been carried out with diploids R vesca , in more indirect ways . Only transfers involv - the modern large - fruited strawberry culti - R viridis , E iinumae , R nubicola , and R ye - ing diploidsand , to a lesser extent , tetraploids vars . Lower ploidy levels are of interest , be - zoensis and with the tetraploid R orientalis . will be discussed here . cause they tend to be highly specialized to of the traits Two approaches have been explored , and Much of the study has involved natural pen - specific environments , and many 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 - E chiloensisand diploidCal - the next higher chromosome level , decaploid ploid California tivars of the future , if they can be introduced ifornia ( 2n = lox = 70 ) through natural doubling R vesca found along the California into the octoploids . of the chromosome number . This has been coast . Only a small part of the rich germplasm accomplished in a number of ways , including : Fra - Intergeneric hybridization between pool available for strawberry breeding has OHybridizing the octoploids with the di - and Potentilla species offers many in - garia been utilized thus far , and genetic accessibil - ploids to produce pentaploids ( 2n = 5 x = teresting possibilities . Two fertile colchicine - ity has been the deciding factor in determin - 35 ) and doubling the chromosome number inducedamphiploidshave been reported , one ing what has been used . The most accessible E chiloensis and with colchicine . species have been American a decaploid hybrid between octoploid R X E virginiana , ancestors of modern cultivated 0Intercrossing or self - pollinatingthe pen - ananassa and P . palustris giving a 14 - ploid ( E X ananassa ) , since all are oc - of = 14x = 98 ) and the other a decaploid strawberries taploids , relying on the higher incidence ( 2n functional unreduced male and female ga - = 8x = 56 chromosomes ) . E chiloensis and toploids ( 2n hybrid between octoploid In interspecific hybridization at the octo - diploid P . glandulosa ( 2n = 10x = 70 ) . The metes to yield decaploids . OUsing natural tetraploids or induced te - ploid level , latter decaploid is of particular interest , be - R chiloensis has been the source traploids ( from diploids ) to hybridize with of the genes presently exploited in all I ; : cause it can be crossed with the decaploid of many important cultivars , including those govern - = chiloensis and R X ananassa x E vesca hy - the octoploids to produce hexaploids ( 2n brids , opening the way for possible transfer ing disease resistance and those that condi - 6 x = 42 ) , and in turn backcrossingthe hex - tion ability to grow vigorously during the aploids to the octoploid , again relying on the Potentilla genes to octoploid strawberries of high incidence of unreduced gametes in the via backcrossing with the resulting 9 - ploid winter . The latter probably trace back to Et - hybrid . R chiloensis terâ??s â?? Fendalcino â?? , a California hexaploids to yield decaploids . = 1 6 ~ = ODeveloping 16 - ploids ( 2n derivativeand ancestor of â?? Lassen â?? , the culti - Royce S . Bringhurst , Professor , Pomology , 112 ) by colchicinetreatment of octoploid ga - var upon which the southern Calfornia com - U.C . , Davis , and Victor Voth , Pomologist , metes in hybridization with the above - men - mercial strawberry industry was built . Davis , stationed at the South Coast Field Station , Santa Ana . I ? virginiana has also been the source of tioned tetraploids . CALIFORNIA AGRICULTURE , AUGUST 1982 25