Correlates of hybridization in plants: long lives, woody growth, and pollination methods are associated with interspecies breeding

A new study in Evol­u­tion Let­ters sheds light on why cer­tain spe­cies are more likely to hybrid­ize than oth­ers. Here, lead author Dr. Nora Mitchell explains her findings.

The phe­nomen­on of hybrid­iz­a­tion has long-fas­cin­ated bot­an­ists and is a primary research focus in our lab group. Plants are notori­ous for their often-promis­cu­ous mat­ing habits and abil­ity to pro­duce viable off­spring with oth­er spe­cies. Invest­ig­a­tions into spe­cif­ic lin­eages have doc­u­mented the ways in which hybrid­iz­a­tion can con­trib­ute to evol­u­tion­ary pro­cesses such as adap­tion, spe­ci­ation, and bio­lo­gic­al inva­sion, but these examples may not be rep­res­ent­at­ive of all plants. Some plant groups tend to hybrid­ize fre­quently, while oth­er groups tend to hybrid­ize rarely or not at all. There is a lack of inform­a­tion regard­ing why plant groups hybrid­ize at dif­fer­ent rates, espe­cially at the glob­al scale. For instance, do plants with cer­tain life his­tory strategies or pol­lin­a­tion syn­dromes tend to hybrid­ize more fre­quently than oth­ers? In a new study in Evol­u­tion Let­ters, we looked for pat­terns between rates of hybrid­iz­a­tion and elev­en dif­fer­ent traits in plants across the world.

sunflower
Case stud­ies of hybrid­iz­a­tion in plants can provide valu­able inform­a­tion on the impact of hybrid­iz­a­tion on pro­cesses such as adapt­ive evol­u­tion, but stud­ies across plants can provide broad­er pic­tures. Image: Sun­flower hybrid. Cred­it: Nora Mitchell.

There are an estim­ated 400,000 spe­cies of vas­cu­lar plants (those with a developed trans­port sys­tem, includ­ing ferns, con­ifers, and flower­ing plants), so it was not logist­ic­ally feas­ible to col­lect data on each spe­cies. Instead, we har­nessed the power of flor­as (pub­lished works that doc­u­ment plant life) to ana­lyze a sub­set of regions. We scanned flor­as from North Amer­ica, the Pacific Islands (Hawai’i), Europe, and Aus­tralia, and coun­ted the num­ber of doc­u­mented cases of hybrids and non­hybrid spe­cies found in each region. Using these data, we cal­cu­lated two hybrid indices based on the num­ber of hybrids rel­at­ive to non­hybrids at the genus and fam­ily levels. We also gathered inform­a­tion on elev­en dif­fer­ent traits with either the­or­et­ic­al or empir­ic­al evid­ence relat­ing them to hybrid­iz­a­tion, includ­ing aspects of life his­tory, growth form, repro­duc­tion, envir­on­ment, and gen­ome size, from the flor­as and addi­tion­al sources.

In total, we col­lec­ted hybrid data and trait inform­a­tion on 195 vas­cu­lar plant fam­il­ies and 1772 vas­cu­lar plant gen­era. Our goal was to see if there were rela­tion­ships between any of our elev­en traits and hybrid­iz­a­tion rates. We incor­por­ated inform­a­tion from pub­lished phylo­gen­et­ic trees to account for related­ness between groups and ran indi­vidu­al mod­els to ana­lyze poten­tial trait × hybrid relationships.

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The region­al flor­as that we ana­lyzed focus on North Amer­ica, but span the globe. Image: Map of regions ana­lyzed. Cred­it: Nora Mitchell.

We found sev­er­al intriguing res­ults using dif­fer­ent com­bin­a­tions of taxo­nom­ic levels and meas­ures of hybrid­iz­a­tion. Our most strik­ing find­ing was that plant groups dom­in­ated by per­en­ni­al spe­cies (those that live for more than one year) ten­ded to have more hybrids than those dom­in­ated by annu­al spe­cies (those that live a year or less). Addi­tion­ally, groups with woody growth forms ten­ded to have more hybrids, and some­times groups with non-liv­ing pol­lin­at­ors and high­er rates of out­cross­ing also ten­ded to have more hybrids.

We pro­posed that these asso­ci­ations could be due to some com­bin­a­tion of factors that pro­mote the form­a­tion of hybrids or factors that pro­mote the per­sist­ence of hybrids. For instance, plants that dis­perse their pol­len via the wind may have a bet­ter chance of mat­ing with a dif­fer­ent spe­cies, enabling them to form hybrids, while those that rely on insects may not have as many oppor­tun­it­ies to do so. Like­wise, long lifespans in per­en­ni­al groups may allow them to have high repro­duct­ive out­put over time des­pite being par­tially sterile, enabling the hybrid lin­eages to per­sist. Although these obser­va­tion­al pat­terns do not imply a caus­al mech­an­ism, they can serve as a basis for fur­ther exper­i­ment­al work invest­ig­at­ing how and why these traits are linked to hybrid­iz­a­tion, and their evol­u­tion­ary con­sequences both in the past and in the future in a chan­ging world.

 

Dr. Nora Mitchell is Assist­ant Pro­fess­or at the Uni­ver­sity of Wis­con­sin – Eau Claire. The ori­gin­al art­icle is freely avail­able to read and down­load from Evol­u­tion Letters.