Researchers Work to Reverse Genetic Erosion, Breed Resilient Olive Varieties

At the cen­ter of the global dis­cus­sion around the link between sus­tain­able devel­op­ment and cli­mate change, bio­di­ver­sity is gen­er­at­ing more inter­est. 

In terms of olive bio­di­ver­sity, recent years have seen a growth of atten­tion among oper­a­tors in the agri-food sec­tor and researchers, who are com­mit­ted to coun­ter­act­ing genetic ero­sion, as well as to study­ing and breed­ing more resilient vari­eties.

“The olive tree (Olea europaea subsp. europaea) is a very ancient tree species with a great wealth of genetic diver­sity,” said Samanta Zelasco, a researcher at the Center for Olive, Fruit and Citrus Crops of the Council for Agricultural Research and Agricultural Economics Analysis Research (CREA-OFA) of Rende, in Calabria.

“First, we must clar­ify that ​‘genetic diver­sity’ is the sci­en­tific def­i­n­i­tion that in the pub­lic debate is gen­er­ally sub­sti­tuted by ​‘bio­di­ver­sity,’” she added. 

According to the last offi­cial cal­cu­la­tion car­ried out by CREA in 2012 on the United Nations Food and Agriculture Organization’s Second Report on the State of Plant Genetic Resources for Food and Agriculture, the olive germplasm present in the major ex situ col­lec­tions in the world amounted to at least 2,629 dif­fer­ent vari­eties. 

Italy has the rich­est genetic diver­sity, with 734 denom­i­na­tions listed in the national reg­is­ter of fruit plant vari­eties, updated in 2020 by the Italian Agriculture Ministry.

“Italy boasts a very high num­ber of vari­eties, and the com­po­si­tion of its olive germplasm is the largest and most var­ied in the world,” Zelasco said. ​“Being in the cen­ter of the Mediterranean, over mil­len­nia, the coun­try has under­gone sev­eral his­tor­i­cal events and peo­ple move­ments that have favored the impor­ta­tion and exchange of plant mate­r­ial, con­tribut­ing to enrich­ing its vari­etal her­itage.”

Today, fre­quent reports of allegedly new geno­types indi­cate a great germplasm diver­sity. Yet, researchers warn that the pre­cise num­ber of vari­eties can­not be defined with cer­tainty due to mul­ti­ple cases of syn­onymy and homonymy.

“We recently car­ried out an in-depth mol­e­c­u­lar inves­ti­ga­tion of the Italian vari­eties using a very large num­ber of mark­ers that cover almost the entire genome,” Zelasco said. ​“We have not com­pleted the study yet, but we can already say that pre­sum­ably a good part of the genetic mate­r­ial, per­haps half, is rep­re­sented by cases of syn­onymy.” 

“The same prob­a­bly hap­pens in other coun­tries,” she added. ​“Still, Italy has a huge num­ber of vari­eties, which are the result of local selec­tions. In most Italian regions, we can find about 30 to 40 vari­eties, then a wide germplasm also at a local level.”

Several lines of sci­en­tific evi­dence indi­cate that the domes­ti­ca­tion area of the olive tree is the Levant (i.e., the Eastern Mediterranean region that includes present-day Cyprus, Israel, Jordan, Lebanon, Syria, Palestine and most of Turkey), from which it was spread around the Mediterranean basin. 

The veg­e­ta­tive prop­a­ga­tion played a cru­cial role, for which rea­son a lot of syn­onymy cases can be found in olive germplasm – Zelasco pro­vides the exam­ple of the Tuscan vari­ety Santa Caterina that shows the same mol­e­c­u­lar pro­file of the Spanish cul­ti­var Gordal Sevillana. Meanwhile, the olive tree has dif­fer­en­ti­ated locally, gen­er­at­ing many vari­eties.

“When cases of syn­onymy, namely dif­fer­ent denom­i­na­tions of the same cul­ti­var, come into play, there may be com­mer­cial prob­lems,” Zelasco said. ​“Legal dis­putes relat­ing to the sourc­ing of table olives and extra vir­gin olive oils from dif­fer­ent geo­graph­i­cal sites than those envis­aged by the Protected Geographical Indications (PGI) and Protected Designation of Origin (PDO) pro­duc­tion reg­u­la­tions are increas­ingly fre­quent.” 

“The rese­quenc­ing of the genomes of the vari­eties included in the spec­i­fi­ca­tions could help to select cul­ti­var-spe­cific mark­ers able to unequiv­o­cally iden­tify the sin­gle vari­eties and pro­vide a more effec­tive genetic cer­ti­fi­ca­tion sys­tem,” she added.

In the cur­rent con­text, where research aims to iden­tify solu­tions in the field of sus­tain­abil­ity, a rich and var­ied gene pool can become use­ful also to draw genes for genetic improve­ment pro­grams.

“A wide bio­di­ver­sity is use­ful for researchers who want to carry out new breed­ing pro­grams,” Zelasco said. ​“The first step is to under­stand the behav­ior of a vari­ety at an agro­nomic level, to detect the genes in its DNA which improve par­tic­u­lar traits, or rather the muta­tions within the genes respon­si­ble for the improve­ment of the agro­nomic traits.”

To observe the genetic expres­sion of diverse vari­eties, their behav­ior must be stud­ied in a sin­gle envi­ron­ment, hence the impor­tance of col­lec­tions.

“It takes years to phe­no­type and char­ac­ter­ize a vari­ety since we must con­sider ​‘on-years’ and ​‘off-years’ in the olive tree’s alter­nate bear­ing cycle, and we need robust data repeated at least over three or four years,” Zelasco said. 

“Since most of the agro­nomic traits are con­di­tioned by the envi­ron­ment, the vari­etal com­par­i­son must also be extended to other envi­ron­ments that can be pre­cisely rep­re­sented by the col­lec­tions con­tain­ing com­mon vari­eties,” she added. ​“Once the com­par­i­son among vari­eties has been con­cluded, we can trace what genes are involved and the muta­tions which are respon­si­ble for the improv­ing traits.”

Currently, such muta­tions are used in the inno­v­a­tive genomic tech­nique called genome edit­ing, which is part of the assisted evo­lu­tion tech­niques, the lat­est nov­elty in the sec­tor. Genome edit­ing con­sists of a punc­tual mod­i­fi­ca­tion of the genome in a spe­cific site, dif­fer­ing from the tra­di­tional muta­ge­n­e­sis that acts ran­domly.

“The improv­ing muta­tions are iden­ti­fied through a sta­tis­ti­cal approach, and we do not trans­fer the DNA, but we iden­tify the sequences and give instruc­tion to an enzy­matic com­plex which is able to repro­duce the muta­tion,” Zelasco said. 

“This is a clear exam­ple of enhanc­ing the genetic diver­sity of the olive tree,” she added. ​“Namely, we iden­tify the improv­ing traits and repro­duce them in the vari­ety to be improved with­out alter­ing its genetic back­ground.”

To take one con­crete exam­ple, Carolea, the most wide­spread vari­ety in Calabria, is one of the cul­ti­vars stud­ied by CREA researchers for its sus­cep­ti­bil­ity to Spilocaea oleaginea, which causes the pea­cock spot dis­ease, a poten­tially sig­nif­i­cant phy­tosan­i­tary prob­lem.

“Recently, we have phe­no­typed the sus­cep­ti­bil­ity to Spilocaea oleaginea of at least 150 vari­eties in our col­lec­tion, and there­fore we will soon be able to iden­tify the improv­ing muta­tions by relat­ing the geno­type to the phe­no­type,” Zelasco said. 

“The genes respon­si­ble for the plan­t’s response to the pathogen have also recently been iden­ti­fied by the research group of the University of Palermo,” she added. ​“Then, in the next few years, we will prob­a­bly be able to repro­duce the improv­ing muta­tion in this vari­ety through the genome edit­ing tech­nique and there­fore to have an improved Carolea that tol­er­ates pea­cock spot.”

Considering the reduc­tion of phy­tosan­i­tary treat­ments with plants less sus­cep­ti­ble to cer­tain dis­eases, research on inno­v­a­tive genomic tech­niques pave the way for the selec­tion of more plants suit­able for new cli­matic sce­nar­ios.

Finally, the study of germplasm makes it pos­si­ble to redis­cover vari­eties that had been set aside and which have char­ac­ter­is­tics that can prove to be use­ful in the cur­rent con­text. 

“I can bring the exam­ple of a neglected Apulian vari­ety called Toscanina, which has a very high con­tent of oleic acid and phe­no­lic com­pounds and enters pro­duc­tion early while being quite pro­duc­tive with a mod­er­ate alter­na­tion,” Zelasco said. 

“We are now test­ing it for tol­er­ance to water stress and biotic stress,” she added. ​“This kind of infor­ma­tion is still miss­ing since these aspects have recently become a pri­or­ity for the sci­en­tific world.”

At the time of writ­ing, five new acces­sions are ready to be included in the CREA’s col­lec­tion in Mirto Crosia, located on the Ionic coast of Calabria, in the province of Cosenza – hav­ing been genet­i­cally char­ac­ter­ized, the acces­sions have unique mol­e­c­u­lar pro­files.

“These are still unpub­lished acces­sions, there­fore poten­tial new vari­eties, which do not have a defined name yet,” Zelasco said. 

She spec­i­fied that they had been found in Calabria and other Italian regions, and some of them come from root suck­ers of cen­turies-old, mon­u­men­tal plants, includ­ing one from the root­stock of the Olivone of Fibbianello in Semproniano, Tuscany.

“Monumental olive trees are an inter­est­ing object of study to under­stand the phy­lo­ge­n­e­sis of the species,” Zelasco said. ​“A very thor­ough genomic analy­sis allows us to under­stand how the species has evolved from the domes­ti­ca­tion phase to date.”

“A cur­rent trend is indeed to cre­ate col­lec­tions of wild plants, and a new col­lec­tion of this kind has been recently cre­ated in Croatia,” she added. ​“We must con­sider that wild olive mate­r­ial is a reser­voir of new genes very use­ful for a breed­ing pro­gram, espe­cially for iden­ti­fy­ing more resilient agro­nomic traits.”