Fruit flies exposed to chronic undernutrition evolve unexpected changes in amino acid metabolism

Post by Fanny Cav­igli­asso and Tadeusz Kawecki

A recent study pub­lished in Evol­u­tion Let­ters invest­ig­ates the impact of eco­lo­gic­al selec­tion pres­sures on thermal plastic responses in growth sched­ules in insect. Authors Fanny Cav­igli­asso and Tadeusz Kawecki tell us more: 

Peri­ods of fam­ine are a fact of life for many anim­al spe­cies, includ­ing our own ancest­ors. The res­ult­ing nat­ur­al selec­tion has favored the tend­ency to set aside energy reserves in the form of body fat, which can be drawn upon when food becomes scarce. Fur­ther­more, when faced with nutri­ent short­ages, many anim­als shut down repro­duc­tion (an example being under­nu­tri­tion-induced amen­or­rhea in humans), an adapt­ive strategy that max­im­izes life­time Dar­wini­an fit­ness by pro­mot­ing imme­di­ate sur­viv­al and the chance to breed when food again becomes avail­able. How­ever, these options are not avail­able to juven­iles, which in most spe­cies can­not sus­pend their devel­op­ment and wait out the dif­fi­cult times. Rather, they must con­tin­ue their devel­op­ment and growth, pro­cesses lim­ited by the avail­ab­il­ity of pro­tein which the body can­not store as it does with fat. Thus, under­nu­tri­tion at a juven­ile stage typ­ic­ally res­ults in stunt­ing and has lifelong health and fit­ness consequences.

Can anim­al physiology evolve in ways that would help juven­iles to grow and devel­op under chron­ic nutri­ent short­age? What changes in the use of the scarce nutri­ents would this involve? To find out, we bred genet­ic­ally vari­able pop­u­la­tions of fruit flies (Dro­so­phila melano­gaster) on a nutri­ent-poor lar­val diet for more than 240 gen­er­a­tions. When con­fron­ted with the poor diet for the first time, Dro­so­phila lar­vae take twice as long to devel­op as they do on stand­ard lab diet, yet the res­ult­ing adults barely attain half of the nor­mal body weight. In the course of evol­u­tion in our exper­i­ment, pop­u­la­tions became more tol­er­ant to the poor diet, attain­ing faster lar­val growth and cut­ting the devel­op­ment­al delay by half. They achieved this tol­er­ance in part by becom­ing more effi­cient at extract­ing and absorb­ing scarce amino acids from the poor diet. Yet, when we examined their lar­val meta­bolome (the abund­ance of diverse meta­bol­ites), we found that they har­bor lower levels of many amino acids in their sys­tem, com­pared to con­trol lar­vae exper­i­en­cing the poor diet for the first time. This is par­tic­u­larly the case for the branched chain amino acids (leu­cine, valine and iso­leu­cine), a trio of struc­tur­ally related essen­tial amino acids that togeth­er account for about 20% of pro­tein con­tent. The levels of free amino acids are sup­posed to reflect the nutri­tion­al status of the organ­ism; in par­tic­u­lar, low levels of leu­cine are thought to sig­nal nutri­tion­al depriva­tion, lead­ing to inhib­i­tion of pro­tein syn­thes­is and growth. Yet our mal­nu­tri­tion-adap­ted lar­vae are nutri­tion­ally bet­ter off and grow faster on the poor diet than non-adap­ted con­trols. So, what is going on?

To gain some under­stand­ing, one should first note that the free amino acids do not in them­selves play a major func­tion­al role – their prin­cip­al use is as build­ing blocks in pro­tein syn­thes­is, fuel­ing organis­mal growth. Thus, their reduced con­cen­tra­tion in mal­nu­tri­tion-adap­ted lar­vae, des­pite faster absorp­tion from poor diet and faster lar­val growth, is sug­gest­ive of more effi­cient cel­lu­lar logist­ics, whereby raw mater­i­als for growth are used as soon as they are avail­able. By ana­logy, stacks of build­ing mater­i­al lay­ing around a con­struc­tion site sug­gest poor organ­iz­a­tion or labor bot­tle­necks rather than a high speed of con­struc­tion; an effi­cient com­pany will make sure that mater­i­als are delivered when needed and used immediately.

How­ever, amino acids can also be used as a source of energy. The first step in this pro­cess is deam­in­a­tion, i.e., remov­al the NH₃ group for ulti­mate excre­tion as a waste product (in the form of urea in mam­mals, uric acid in insects, birds and rep­tiles). The remain­ing carboxyl­ic skel­et­on can then be dir­ectly used to gen­er­ate ATP (the cel­lu­lar energy car­ri­er) or con­ver­ted to gly­co­gen or fat as energy stor­age. Obvi­ously, lar­vae need energy for growth, for­aging and oth­er life-sus­tain­ing pro­cesses, and they must also store some fat to fuel the ener­get­ic­ally costly pro­cess of meta­morph­os­is. “Burn­ing” the spare amino acids, how­ever, seems waste­ful, espe­cially giv­en that we have shown that lar­val growth on poor diet is lim­ited by diet­ary pro­tein rather than carbohydrates.

We there­fore hypo­thes­ized that the mal­nu­tri­tion-adap­ted lar­vae would min­im­ize “burn­ing” of amino acids and instead derive all their energy from diet­ary car­bo­hydrates (from corn­meal and sug­ar con­tained in the food medi­um). To test this hypo­thes­is, we exploited the fact that the deam­in­a­tion reac­tions have a slight pref­er­ence for NH₃ groups con­tain­ing the com­mon ¹⁴N iso­tope. As a con­sequence, grow­ing lar­val tis­sues accu­mu­late the heav­ier stable iso­tope ¹⁵N in pro­por­tion to the frac­tion of amino acids that are deam­in­ated with their nitro­gen­ous waste product being excreted. Thus, we expec­ted that the mal­nu­tri­tion-adap­ted lar­vae would show a lower ¹⁵N/¹⁴N ratio at the end of their devel­op­ment than lar­vae from con­trol pop­u­la­tions. But we found the oppos­ite: while the ¹⁵N/¹⁴N ratio in con­trol lar­vae raised on poor diet was indis­tin­guish­able from the ratio in the diet, mal­nu­tri­tion-adap­ted lar­vae showed elev­ated levels of ¹⁵N, although not as high as lar­vae raised on pro­tein-rich diets. This implies that while lar­vae with no (recent) evol­u­tion­ary his­tory of under­nu­tri­tion respond to it by hoard­ing amino acids, evol­u­tion­ary adapt­a­tion favored a lim­ited use of amino acids as energy source des­pite their scarcity. This con­clu­sion is cor­rob­or­ated by dif­fer­ences in the levels of the nitro­gen­ous waste product uric acid.

To inter­pret this unex­pec­ted res­ults, it helps to know that although the mal­nu­tri­tion-adap­ted lar­vae are bet­ter at extract­ing amino acids from their poor diet, they are less effi­cient at extract­ing car­bo­hydrates, an appar­ent trade-off in acquis­i­tion of scarce mac­ronu­tri­ents. Thus, not only can they afford to “burn” some amino acids; they may be com­pelled to do so by not obtain­ing enough diet­ary carbohydrates.

One para­dox remains unre­solved. The rate of pro­tein syn­thes­is in cells is reg­u­lated by the TOR sig­nal­ing path­way based on the levels of free amino acids, in par­tic­u­lar leu­cine. This pre­vents cells from ini­ti­at­ing the syn­thes­is of more pro­tein molecules than it has amino acids to com­plete. The low level of leu­cine in the mal­nu­tri­tion-adap­ted lar­vae should thus inhib­it pro­tein syn­thes­is and organis­mal growth by sup­press­ing the TOR path­way. This does not seem to hap­pen, sug­gest­ing that the evol­u­tion­ary adapt­a­tion to nutri­ent short­age in our evol­u­tion exper­i­ment also involved changes in the TOR path­way and/or oth­er mech­an­isms that reg­u­late cel­lu­lar growth and divi­sion in response to nutri­tion­al situ­ation. This is one of the dir­ec­tions we are now exploring.

Our study demon­strates that the meta­bol­ism of juven­ile anim­als can rap­idly evolve to alle­vi­ate con­sequences of nutri­ent short­age for growth and devel­op­ment; physiolo­gic­al adapt­a­tion to fam­ine is thus not lim­ited to stor­ing fat and shut­ting down repro­duc­tion. It remains to be seen what side effects and trade-offs such meta­bol­ic adapt­a­tion may entail, but our res­ults boost the idea that aspects of human meta­bol­ism that make us vul­ner­able to dis­ease may have been favored by nat­ur­al selec­tion in our ancest­ors dur­ing peri­ods of famine.

The Kawecki group is loc­ated in the Depart­ment of Eco­logy and Evol­u­tion at the Uni­ver­sity of Lausanne. The ori­gin­al art­icle is freely avail­able to read and down­load from Evol­u­tion Letters.