Stearns Graduate Student Prize

I am inter­ested in pat­terns of repro­duct­ive isol­a­tion across pop­u­la­tions and spe­cies groups, includ­ing how and why they vary. Mon­key­flowers are a great and diverse sys­tem to study repro­duct­ive isol­a­tion, but most work in the group has focused on just a few mod­el taxa. 

This study, con­duc­ted as part of my Ph.D. with advisor Dr. Andrea Swe­igart at the Uni­ver­sity of Geor­gia, arose out of a col­lab­or­at­ive effort to expand our under­stand­ing of repro­duct­ive isol­a­tion across a lar­ger num­ber of mon­key­flower spe­cies groups. We chose a group of spe­cies with very little pre­vi­ous inform­a­tion, and char­ac­ter­ized pat­terns of gen­om­ic diver­gence and diversity, post­mat­ing repro­duct­ive isol­a­tion, and poten­tial hybrid­iz­a­tion. We found that repro­duct­ive isol­a­tion in this group, par­tic­u­larly hybrid seed invi­ab­il­ity, is very strong and pre­vents ongo­ing gene flow, though we do detect sig­nals of his­tor­ic­al gene flow in the group. Hybrid seed invi­ab­il­ity in this group is asso­ci­ated with dif­fer­ences in par­ent­al seed size, in con­trast to oth­er well-stud­ied cases of seed invi­ab­il­ity in mon­key­flowers, which may implic­ate selec­tion on seed size as an indir­ect driver of repro­duct­ive isolation. 

I went to Duke Uni­ver­sity for my under­gradu­ate edu­ca­tion, where I fell in love with both wild­flowers and evol­u­tion­ary genet­ics. After gradu­at­ing, I worked as a labor­at­ory tech­ni­cian in a Phlox evol­u­tion lab with Dr. Robin Hop­kins at Har­vard Uni­ver­sity, before start­ing my Ph.D. work at the Uni­ver­sity of Geor­gia. In addi­tion to this study, my Ph.D. work has examined how repro­duct­ive isol­a­tion and admix­ture pat­terns vary across space, time, and the gen­ome with­in pop­u­la­tions of hybrid­iz­ing monkeyflowers.

I am hon­oured to receive a Ste­arns Prize for this research, which was con­duc­ted dur­ing my PhD research at the Uni­ver­sity of Toronto, Canada with my advisor Dr. Megan Fre­d­er­ick­son and sev­er­al won­der­ful under­gradu­ate co-authors. 

This research paper stemmed from a need to bet­ter under­stand the con­sequences of the diversity and abund­ance of mutu­al­isms plants engage in in nature. Mul­tiple mutu­al­isms, asso­ci­ations between a single host and mul­tiple part­ner spe­cies, are ubi­quit­ous in plants, which often asso­ci­ate with pol­lin­at­ing, seed-dis­pers­ing, and defens­ive arth­ro­pod mutu­al­ists. These part­ners can impose con­flict­ing selec­tion pres­sures that alter their focal host’s evol­u­tion­ary trajectory. 

Here, I set out to invest­ig­ate genet­ic cor­rel­a­tions among traits asso­ci­ated with mat­ing sys­tem, biot­ic defence, pol­lin­a­tion, and seed dis­pers­al mutu­al­ism in the trop­ic­al weed Turn­era ulmi­fo­lia. Using just under 200 T. ulmi­fo­lia gen­o­types col­lec­ted from across Jamaica, we fit genet­ic vari­ance-cov­ari­ance (G) matrices to mutu­al­ist­ic and repro­duct­ive trait data using Bayesian meth­ods and found sig­ni­fic­ant pos­it­ive genet­ic cor­rel­a­tions among traits asso­ci­ated with out-cross­ing, pol­lin­a­tion, and biot­ic defence. These pat­terns are con­sist­ent with genet­ic facil­it­a­tion in the evol­u­tion of plant-arth­ro­pod mutu­al­isms, and hint at pat­terns of cor­rel­ated selec­tion on flor­al mor­pho­logy, pol­lin­a­tion, and the defence of key plant tis­sues. Assess­ing vari­ation in G at a loc­al scale using our largest pop­u­la­tions, we found few dif­fer­ences in the mag­nitude and ori­ent­a­tion of G, and that loc­al dif­fer­ences were con­sist­ent with diver­gence among the genet­ic lines of least resistance.

This study mainly explores the impact of inter­spe­cif­ic inter­ac­tions on the dif­fer­en­ti­ation of pop­u­la­tions with­in a spe­cies. For this reas­on, I selec­ted two ground beetle spe­cies that are exper­i­en­cing sec­ond­ary con­tact as the mod­el for this study. The two spe­cies are Car­abus maiy­as­anus and C. iwawaki­anus, which are well known to have the gen­it­al lock-and-key system. 

I espe­cially focused on the former spe­cies that rep­res­ents dif­fer­en­ti­ation in gen­it­al mor­pho­lo­gies among pop­u­la­tions (i.e., repro­duct­ive char­ac­ter dis­place­ment, pos­sibly via rein­force­ment), and I wondered this will lead to the ini­tial spe­ci­ation with­in the spe­cies (the cas­cade rein­force­ment hypo­thes­is). This study provides some empir­ic­al sup­port for this hypo­thes­is, and I hope it can inspire future related research and provide new ideas for vari­ous ways in spe­ci­ation studies.

This paper was inspired by a desire to under­stand the select­ive pres­sures driv­ing col­our pat­tern diver­gence among closely related, sym­patric spe­cies and I am hon­oured that it has been recog­nized with a Ste­arns Prize. This pro­ject was con­duc­ted as part of my PhD research at Queen’s Uni­ver­sity in Ontario, Canada in col­lab­or­a­tion with my PhD advisor Paul R. Martin. 

To study beha­vi­our­al responses to col­our pat­tern dif­fer­ences between spe­cies, inde­pend­ent of size or shape dif­fer­ences, I painted 3D prin­ted mod­els to exactly match the spec­tro­met­er-meas­ured col­ours of indi­vidu­al male museum spe­ci­mens of three bird spe­cies: black-capped chickadees (Poe­cile atri­ca­pil­lus) and their equally closely related con­gen­ers, moun­tain chickadees (P. gam­beli) and Mex­ic­an chickadees (P. sclateri). Black-capped chickadees co-occur with the more dif­fer­ently col­oured moun­tain chickadee in parts of their range, but nev­er live with the more sim­il­arly col­oured Mex­ic­an chickadee. To test the hypo­thes­is that these col­our pat­tern dif­fer­ences are driv­en by selec­tion against hybrid­iz­a­tion, we presen­ted pairs of these mod­els to naïve black-capped chickadee females and observed which mod­els they were most likely to dir­ect cop­u­la­tion soli­cit­a­tion dis­plays towards. We found that females were less inter­ested in mat­ing with more diver­gently col­oured mod­els under cer­tain con­di­tions, sug­gest­ing that col­our pat­tern diver­gence may indeed reduce mixed mat­ing, and should there­fore be favoured by selec­tion against hybrid­iz­a­tion. This exper­i­ment was extremely chal­len­ging, but incred­ibly reward­ing, and I am excited about what else we can learn about sig­nal evol­u­tion and spe­cies coex­ist­ence through this type of tightly con­trolled field exper­i­ment in the future.

I majored in zoology dur­ing my bachelor’s and master’s degrees at the Pres­id­ency Uni­ver­sity, Kolk­ata, India, with a spe­cial­iz­a­tion in eco­logy. Then I moved to Switzer­land to pur­sue my PhD in evol­u­tion­ary bio­logy in the lab of Prof. Rolf Küm­merli at the Uni­ver­sity of Zurich study­ing phen­o­typ­ic het­ero­gen­eity in bac­teri­al pop­u­la­tions. There­after, I pur­sued postdoc­tor­al research study­ing prey-pred­at­or inter­ac­tions in microbes. Recently I have moved to the Uni­ver­sity of Pennsylvania, USA for a postdoc in the field of immune-micro­bi­o­me inter­ac­tions. I am broadly inter­ested in field of micro­bi­al eco­logy and evol­u­tion, with the inten­tion to bridge fun­da­ment­al and trans­la­tion­al research.

In this research con­duc­ted dur­ing my PhD, we invest­ig­ated wheth­er divi­sion of labour evolve with respect to pub­lic goods in the oppor­tun­ist­ic patho­gen­ic social bac­teri­um Pseudo­mo­nas aer­u­ginosa. We observed that spe­cial­ists did not indulge in cooper­at­ive divi­sion of labour but rather could co-exist via mutu­al cheat­ing. Con­trary to pop­u­lar obser­va­tions in oth­er study sys­tems, our res­ults sug­gest that there is a nar­row range of con­di­tions under which divi­sion of labour can evolve.

(X handle: @SubhamMridha)

This study was con­duc­ted as part of my PhD research at the Uni­ver­sity of Salford and the Zoolo­gic­al Soci­ety of Lon­don. Under the guid­ance of my super­visors Robert Jehle and Trent Garner, along­side col­leagues at the Insti­tute of Zoology and the High­land Amphi­bi­an and Rep­tile Pro­ject, I set out to explore the rela­tion­ship between host genet­ic diversity and capa­city to defend against nov­el patho­gens. Here, the host was the com­mon toad, and the patho­gen Bat­ra­chochytri­um dendroba­tid­is (Bd), a fungus that has caused cata­stroph­ic declines and extinc­tions across hun­dreds of amphi­bi­an spe­cies, mak­ing it the most destruct­ive patho­gen of ver­teb­rates ever char­ac­ter­ised. It is not, how­ever, uni­ver­sally destruct­ive, and some host indi­vidu­als and pop­u­la­tions fare bet­ter than oth­ers. To bet­ter under­stand this, we focused on toad pop­u­la­tions situ­ated on the north-west­ern edge of the spe­cies range on and around the isle of Skye in Scotland.

Com­bin­ing con­trolled infec­tion exper­i­ments with genet­ic ana­lyses, we showed con­sid­er­able genet­ic diver­gence between toad pop­u­la­tions and a strong influ­ence of pop­u­la­tion iden­tity on response to this nov­el patho­gen. But this response did not seem to be driv­en by genet­ic erosion on com­par­at­ively isol­ated islands. Indeed, het­ero­zy­gos­ity showed an unex­pec­ted neg­at­ive rela­tion­ship with sur­viv­al. Our find­ings under­score the import­ance of con­text depend­ency in com­plex host-patho­gen dynam­ics, and cau­tion against simplist­ic assump­tions about the vul­ner­ab­il­ity of genet­ic­ally depau­per­ate host populations.

These days, I still focus on islands, but in a dif­fer­ent way. I work with Mon­ash Uni­ver­sity research­ing and restor­ing the eco­logy of Browse Island in the Timor Sea, help­ing a degraded island once teem­ing with seabirds to flour­ish again.