Solving the mystery of increasing seed sizes

By Megan Widdows

Veget­ables and crops such as wheat and rice are some of the most import­ant plants in our food pro­duc­tion. But have they always had the same char­ac­ter­ist­ics that they have today? 

Not at all. In fact, the ancest­ors of the crops we farm today were very dif­fer­ent and over time they have changed to be more suit­able for farm­ing and human con­sump­tion. One of the defin­ing char­ac­ter­ist­ics of crops becom­ing domest­ic­ated, in the Neo­lith­ic peri­od and bey­ond, was the loss of nat­ur­al seed dis­pers­al meth­ods as plants became reli­ant on farm­ers to sow the seeds each year. Anoth­er change that occurred around this time was seed enlargement. 

But how did these changes occur? Changes to a spe­cies over time are pos­sible due to nat­ur­al selec­tion, or sur­viv­al of the fit­test, where the organ­isms with the most desir­able char­ac­ter­ist­ics are more likely to sur­vive and repro­duce, passing their genes on to their off­spring. Nat­ur­al selec­tion occurs due to vari­ation with­in pop­u­la­tions of the same species. 

One way of pro­du­cing genet­ic vari­ation is through muta­tions in our DNA. Muta­tions are changes to the DNA sequence that makes up our genes. They occur spon­tan­eously dur­ing the DNA rep­lic­a­tion pro­cess. Most of the time muta­tions don’t change any of the char­ac­ter­ist­ics of an organ­ism, but some­times a muta­tion will be what we call dele­ter­i­ous. This means it will have a neg­at­ive effect on the organ­ism and will reduce the chance of an organ­ism sur­viv­ing and passing on its genes to the next generation.

Altern­at­ively, some­times a muta­tion cre­ates a pos­it­ive change in an organ­ism, for example mak­ing it lar­ger or stronger. This makes an organ­ism more likely to sur­vive and pass on its genes, passing this new muta­tion onto the next generation. 

Nat­ur­al selec­tion is hap­pen­ing all the time in nature, and it is one of the main drivers of evol­u­tion and adapt­a­tion. How­ever, it is not the only evol­u­tion­ary tool for cre­at­ing change with­in a pop­u­la­tion. Ever since the ori­gins of farm­ing, humans have been speed­ing up the pro­cess of nat­ur­al selec­tion by choos­ing anim­als and crops with the most desir­able char­ac­ter­ist­ics to breed. This ensures that future gen­er­a­tions have the genes that encode the most desir­able char­ac­ter­ist­ics, help­ing to improve yield or provide anoth­er advant­age to farm­ers. This pro­cess is known as arti­fi­cial selec­tion

Anoth­er cause of genet­ic changes in a pop­u­la­tion is a pro­cess called uncon­scious selec­tion. In con­trast to arti­fi­cial selec­tion, where farm­ers know­ingly select for desired char­ac­ter­ist­ics, uncon­scious selec­tion occurs when farm­ers unknow­ingly select for a char­ac­ter­ist­ic that they do not care about. This can hap­pen when the char­ac­ter­ist­ic is genet­ic­ally linked to a desir­able char­ac­ter­ist­ic which is being act­ively select­ing for. When organ­isms with the desir­able char­ac­ter­ist­ic are bred, their off­spring inher­it both the selec­ted char­ac­ter­ist­ic and any oth­er char­ac­ter­ist­ics that are genet­ic­ally linked to it. For years, sci­ent­ists have wondered how the veget­ables and crops we eat today have evolved. In 2017, a group of sci­ent­ists at the Uni­ver­sity of Shef­field set out to dis­cov­er the evol­u­tion­ary causes of seed enlarge­ment, one of the key fea­tures of crop domest­ic­a­tion. The seeds of domest­ic cer­eal crops that we eat today, like wheat, rice, and maize, have much lar­ger seeds than their wild relatives.

One hypo­thes­is was that the lar­ger seed size of domest­ic crops could be a res­ult of arti­fi­cial selec­tion, whereby the farm­ers iden­ti­fied large size as a desir­able trait in crop seeds and took steps to enhance and increase this trait in future gen­er­a­tions. Early farm­ers may have believed that lar­ger seeds would bring high­er crop yields, or they may have found lar­ger seeds easi­er to handle and sow, so chose these over smal­ler seeds. 

Altern­at­ively, lar­ger seeds could have aris­en as a res­ult of uncon­scious selec­tion, if the genes respons­ible for seed size were linked to anoth­er char­ac­ter­ist­ic such as crop height. If this were the case, when farm­ers selec­ted for taller crops, they would have also unknow­ingly selec­ted for lar­ger seeds. 

Anoth­er the­ory was that the increase in seed size came about through nat­ur­al selec­tion, as a con­sequence of the changes that happened when humans brought about crop domest­ic­a­tion. It is pos­sible that lar­ger seeds may have increased the chance of the plant sur­viv­ing or reproducing.

To test wheth­er early farm­ers may have unknow­ingly selec­ted for large seed size when breed­ing plants, the sci­ent­ists stud­ied seed size in domest­ic­ated veget­ables and their wild rel­at­ives. Veget­ables were chosen because any delib­er­ate selec­tion for desired char­ac­ter­ist­ics in these plants would have focused on increas­ing the size of their leaves, roots or stems, since these are the parts that are eaten, rather than the seeds. If there has been an increase in seed size in domest­ic­ated veget­ables, it is likely to be the res­ult of uncon­scious or nat­ur­al selection. 

The sci­ent­ists found that, just like cer­eal crops, domest­ic­ated veget­ables have lar­ger seeds than their wild rel­at­ives – sug­gest­ing that arti­fi­cial selec­tion is unlikely to explain large seed size in domest­ic crops. 

This dis­cov­ery is import­ant because it shows us how sig­ni­fic­ant changes to our staple crops and veget­ables were brought about by our early farm­er ancest­ors without their delib­er­ate inten­tion. With­in stud­ies of evol­u­tion, it also high­lights the import­ance of the often over­looked pro­cess of uncon­scious selection. 

The study men­tioned in this art­icle can be found here. A gloss­ary of key terms is provided below.

Tell us what you think about this blog… 

We are try­ing to under­stand who reads our blogs and why, to help us improve their content. 

By com­plet­ing this sur­vey, you agree that you are over the age of 18 and that your responses can be used in research at the Uni­ver­sity of Shef­field to eval­u­ate the effect of blog­ging in sci­ence communication. 

Gloss­ary:

Arti­fi­cial selec­tion – tak­ing steps to enhance and per­petu­ate traits that have been iden­ti­fied as desir­able by humans in future gen­er­a­tions of plants and animals 

Dele­ter­i­ous – some­thing that causes harm or a neg­at­ive change, usu­ally has a neg­at­ive effect on the organ­ism and will reduce the chance of the organ­ism sur­viv­ing and passing on its genes to the next generation.

Evol­u­tion – the gradu­al pro­cess by which liv­ing organ­isms have changed and adap­ted from earli­er forms 

Muta­tions – changes to the struc­ture or make up of a gene that can res­ult in changes to an organ­ism and/or its offspring

Nat­ur­al selec­tion – the pro­cess whereby organ­isms that are bet­ter adap­ted to their envir­on­ment tend to sur­vive and pro­duce more offspring

Uncon­scious selec­tion – non-inten­tion­al human selec­tion that can occur when a char­ac­ter­ist­ic is genet­ic­ally linked to anoth­er char­ac­ter­ist­ic. If one of these char­ac­ter­ist­ics is arti­fi­cially selec­ted, the oth­er will also be unin­ten­tion­ally selected. 

Vari­ation – any dif­fer­ences between cells, organ­isms or groups of organ­isms caused by genet­ic dif­fer­ences or envir­on­ment­al factors.