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Jozef Oboňa & Peter Mikula

Zatiaľ čo mnohí z nás v tejto otázke jasno nemajú, existujú organizmy, ktorým je odpoveď zrejmá už veľmi dlho. Reč však nebude o žiadnych antropocentrických problémoch trápiacich určitú časť ľudstva, ale o drobnej aberantnej skupinke hmyzu. Nasledujúcich pár riadkov bude preto venovaných muchám označovaným akoStalk-eyed flies“, alebo ak chcete – muchám s očami na stopkách. Tieto dvojkrídlovce patria do prevažne tropickej čeľade Diopsidae a tvoria len pomerne malú čeľaď múch o veľkosti len niekoľkých desiatok druhov. Vzhľadom k ich životnému cyklu a spôsobu života je ich výskyt viazaný najmä na vlhké miesta s rozkladajúcou sa vegetáciou napr. v okolí potokov a riek. Jeden zástupca tejto skupiny (vedecky Diopsis macrophthalma), vyskytujúci sa v Afrike, je dokonca považovaný za škodcu na ryžových poliach1. Ak vás ich zovňajšok alebo spôsob života zaujal a chceli by ste ich vidieť na vlastné oči, nie je potrebné cestovať až tak ďaleko, pretože jeden zástupca tejto čeľade bol len pred nedávnom objavený aj v Európe a to dokonca u našich južných susedov v Maďarsku2.

 1Druh patriaci do skupiny druhov Diopsis apicalis z lokality Nanta Mesi, dokumentárne foto.

Tak trochu sme dúfali, že tieto bizarné tvory nájdeme aj na našej nedávnej expedícii v Keni, ale pri pohľade na vyschnuté korytá riek a vyprahnutú krajinu sme začali byť skeptickí. No nakoniec sme uspeli v severokeňskom pohorí Mt. Kulal, na lokalite Nanta Mesi. Je to nevysýchajúce jazierko pod vodopádom, ktoré intenzívne využívajú miestni pastieri ako napájadlo pre dobytok v období sucha. To že sú tu aj muchy s očami na stopkách sme ani netušili, až pokiaľ sa nám jedna z nich nechytila do entomologickej sieťky. A viac nám nebolo treba, celú lokalitu sme podrobne presmýkali s výsledkom – dva odchytené exempláre (teda mohlo to byť aj lepšie…). Jedná sa o dva exempláre toho istého druhu (patriaceho do veľkej skupiny podobných druhov Diopsis apicalis – determinácia : Hans Feijen).

Lokalita Nanta Mesi (Mount Kulal): nevysýchajúce jazierko pod vodopádom predstavuje určitý ostrov pre biodiverzitu vodných živočíchov počas obdobia sucha.

Lokalita Nanta Mesi (Mount Kulal): nevysýchajúce jazierko pod vodopádom predstavuje určitý ostrov pre biodiverzitu vodných živočíchov počas obdobia sucha.

Tieto bizarné tvory sa dajú najjednoduchšie spozorovať za súmraku (aj keď nám sa ich podarilo chytiť na pravé poludnie), kedy sa zoskupujú vždy na rovnakých miestach. Avšak zatiaľ čo samice sa zhlukujú tesne vedľa seba, samce sú rozostúpené a „bojujú“ o svoje miesto pod slnkom (v tomto prípade skôr pod mesiacom). Ide o určitý rituál, pri ktorom si samce navzájom porovnávajú dĺžku svojich očných stopiek a ich rozpätie. V honbe za maximalizovaním rozpätia očných stopiek sa niektoré samce uchyľujú k trikom s vystretými prednými nohami, ktorými opticky ešte predĺžia vzdialenosti medzi oboma očnými stopkami. Využívaniu takýchto „podpásoviek“ sa však nemôžeme čudovať. V stávke je skutočne veľa a postup je v tomto prípade neúprosne jednoduchý a priamočiary – samec s dlhšími očnými stopkami jednoducho vyhráva a získava viac samíc3. U samcov s kratším rozpätím stopiek sú vyhliadky na úspešný vertikálny transfer vlastných génov dosť pochmúrne a známe tvrdenie „aj s malým šaškom sa dá zahrať veľké divadlo“ v tomto prípade nie je na mieste.

Vedci stále uvažujú nad tým, prečo je tomu tak. Výsledky ich bádania ale takmer vždy prinášajú rovnaký výsledok: atraktivita samcov sa zvyšuje s dĺžkou ich očných stopiek. To môže poukazovať nato, že samice prostredníctvom sexuálneho výberu samé podporujú vývoj extrémnej morfológie samčích očných stopiek4. „Chudák samec“, mohli by sme si pomyslieť pri predstave nekonečného tlaku na rast dĺžky očných tykadiel. Na(ne)šťastie proti účinkom pohlavného výberu pôsobí dosť tvrdo výber prirodzený, ktorý prostredníctvom zníženej životaschopnosti (napr. sú dané jedince nemotornejšie alebo viditeľnejšie v dôsledku čoho sa následne stávajú častou korisťou predátorov) eliminuje z populácie jedince s príliš excesivným prejavom daného znaku. V prípade samcov „stalk-eyed fly“ sa, podobne ako aj u iných druhov živočíchov, nejedná len o určitú zlovôľu či tortúru členov mužského pohlavia zo strany samíc. Samičí výber je aj tu vskutku pragmatická záležitosť, pretože sa ukazuje, že s dĺžkou očnej stopky úzko súvisí napríklad celková veľkosť, dĺžka krídla, ako aj vitalita či plodnosť samca. Väčšia dĺžka stopiek teda v konečnom dôsledku znamená väčšiu veľkosť tela a zvýšenú vitalitu otca, čo sa vo výsledku prejaví produkciou zdatnejších a životaschopnejších potomkov5. Vedci naviac zistili, že gén v ktorom je zakódovaná informácia o dlhšej očnej stopke je spojený s génom, ktorý umožňuje splodiť viac samčích potomkov6.

Takže aby sme si to nejako zhrnuli…

Na dĺžke skutočne záleží, aspoň v tomto prípade…

 

 

Táto zahraničná cesta bola realizovaná z projektu: Inovácia vzdelávacieho a výskumného procesu ekológie ako jednej z nosných disciplín vedomostnej spoločnosti (ITMS 26110230119), ktorý je spolufinancovaný zo zdrojov EÚ.

 

 

  1. Alghali A.M. (1984). Mating and ovipositional behavior of the stalk-eyed fly Diopsis macrophthalma on rice. Entomologia Experimentalis et Applicata 36: 151–157.
  2. Papp L., Földvári M. & Paulovics P. (1997). Sphyracephala europaea sp. n. (Diptera: Diopsidae) from Hungary represents a family new to Europe. Folia Entomologica Hungarica 58: 137–146.
  3. Kirkpatrick M. (1982). Sexual selection and the evolution of female choice. Evolution 36: 1–12.
  4. Wilkinson G. & Reillo P. (1994). Female choice response to artificial selection on an exaggerated male trait in a stalk-eyed fly. Proceedings of the Royal Society of London B 255: 1–6.
  5. Zimmer C. (2008). The evolution of extraordinary eyes: the cases of Flatfishes and Stalk-eyed Flies. Evolution Education and Outreach 1: 487. doi:10.1007/s12052-008-0089-9.
  6. David P., Bjorksten T., Fowler K. & Pomiankowski A. (2000). Condition-dependent signalling of genetic variation in stalk-eyed flies. Nature 406: 186–188. doi:10.1038/35018079. PMID10910358.

 

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