{"id":18192,"date":"2017-12-01T18:22:26","date_gmt":"2017-12-01T15:22:26","guid":{"rendered":"http:\/\/109.232.216.219\/~bilimvegelecek\/?p=18192"},"modified":"2018-02-08T16:19:13","modified_gmt":"2018-02-08T13:19:13","slug":"yuksek-adaptasyon-ve-gizli-genetik-varyasyon-1","status":"publish","type":"post","link":"https:\/\/bilimvegelecek.com.tr\/index.php\/2017\/12\/01\/yuksek-adaptasyon-ve-gizli-genetik-varyasyon-1","title":{"rendered":"Y\u00fcksek adaptasyon ve gizli genetik varyasyon – 1"},"content":{"rendered":"

Ara\u015ft\u0131r\u0131c\u0131lar, Limulus, Coelacanth ve di\u011fer \u201cya\u015fayan fosiller\u201din ya\u015fam\u0131n tarihinin \u00e7ok uzun d\u00f6nemleri boyunca de\u011fi\u015fmeden kalm\u0131\u015f g\u00f6z\u00fckmelerini, yery\u00fcz\u00fcn\u00fcn be\u015f b\u00fcy\u00fck kitlesel yok olu\u015fundan en az birinden sa\u011f \u00e7\u0131kan formlar\u0131n, y\u00fcksek adaptasyon g\u00f6stermeleri ve d\u00fc\u015f\u00fck ekolojik de\u011fi\u015fkenlik g\u00f6steren ortamlarda ya\u015fayagelmeleriyle izah ediyorlar.<\/em><\/p>\n

Ge\u00e7ti\u011fimiz ayki yaz\u0131m\u0131zda, \u201cya\u015fayan fosil\u201d olarak tabir edilen Coelecanth\u2019\u0131n, fosil formu ile ya\u015fayan t\u00fcrleri aras\u0131ndaki morfolojik a\u00e7\u0131dan olduk\u00e7a yak\u0131n benzerli\u011fe ra\u011fmen, genom d\u00fczeyinde inildi\u011finde hem kendinden \u00f6nceki ve sonraki canl\u0131larla tarihsel olarak ili\u015fkilendi\u011fini hem de karasal ya\u015fama ge\u00e7i\u015fle ilgili adaptasyonlar konusunda son derece \u00f6nemli bilgiler verdi\u011fini g\u00f6stermeye \u00e7al\u0131\u015fm\u0131\u015ft\u0131k. Coelacanth genomunu konu alan ba\u015fl\u0131ca \u00e7al\u0131\u015fmayla ayn\u0131 y\u0131l i\u00e7erisinde yay\u0131nlanan bir di\u011fer makale de, \u00f6zellikle \u00f6n ve arka uzuvlar\u0131n karasal ya\u015famla de\u011fi\u015fmesi temeline odaklanmakta ve Coeleacanth\u2019\u0131n yava\u015f evrimle\u015fen bir genoma sahip oldu\u011funu bir kez daha vurgulayarak karasal uzuv (ayak) olu\u015fumuma giden tarihsel yoldaki genetik de\u011fi\u015fimler hakk\u0131nda net bilgiler vermektedir.1<\/sup><\/p>\n

Bir di\u011fer \u201cya\u015fayan fosil\u201d: Limulus polyphemus<\/strong><\/p>\n

\u201cYa\u015fayan fosil\u201d ya da \u201cfilogenetik kal\u0131nt\u0131\u201d \u015feklinde tan\u0131nan bir di\u011fer hayvan t\u00fcr\u00fc ise, Kuzey Amerika\u2019n\u0131n Atlantik sahilleri boyunca yay\u0131lan at nal\u0131 yengeci, Limulus polyphemus<\/em>\u2019tur. Bu son derece arkaik g\u00f6r\u00fcn\u00fcml\u00fc enteresan canl\u0131, t\u0131pk\u0131 Coelacanth gibi, 150 milyon y\u0131l\u0131 a\u015fk\u0131n fosil formlar\u0131 ile aras\u0131nda b\u00fcy\u00fck benzerlik ta\u015f\u0131maktad\u0131r ve di\u011fer pek \u00e7ok \u00f6zelliklerinin yan\u0131 s\u0131ra, kan\u0131n\u0131n t\u0131bbi malzemelere bula\u015fm\u0131\u015f gram negatif bakterileri tan\u0131mada kullan\u0131lan en duyarl\u0131 madde olmas\u0131yla da me\u015fhurdur.<\/p>\n

Limulus polyphemus<\/em>, evrimsel biyolojik a\u00e7\u0131dan bir zamanlar hayli tart\u0131\u015fma yaratan ve d\u00fc\u015f\u00fck genetik \u00e7e\u015fitlili\u011fin taksonlar\u0131n evrimsel tarihi boyunca d\u00fc\u015f\u00fck yap\u0131sal ve morfolojik \u00e7e\u015fitlilik g\u00f6stermesinin sebebi oldu\u011funu \u00f6ne s\u00fcren yakla\u015f\u0131m\u0131 da s\u0131namak i\u00e7in kullan\u0131lm\u0131\u015f model bir canl\u0131d\u0131r da. Bu yakla\u015f\u0131ma g\u00f6re, genlerindeki alel (ayn\u0131 bir genin farkl\u0131 \u00f6zellik durumlar\u0131 yaratabilen farkl\u0131 formlar\u0131) say\u0131s\u0131 \u00e7ok d\u00fc\u015f\u00fck olan ve \u00e7ok y\u00fcksek s\u0131kl\u0131\u011fa sahip sadece bir ya da birka\u00e7 alel ile temsil edilen t\u00fcrlerin, evrimsel tarihleri boyunca g\u00f6sterdikleri de\u011fi\u015fim miktarlar\u0131 da (hem yeni t\u00fcrlerin olu\u015fmas\u0131 hem de ayn\u0131 bir t\u00fcrdeki yap\u0131sal-morfolojik \u00e7e\u015fitlenme) \u00e7ok d\u00fc\u015f\u00fck olacakt\u0131r. Yeni t\u00fcrlerin ya da ayn\u0131 t\u00fcr i\u00e7inde morfolojik farkl\u0131l\u0131klar g\u00f6steren formlar\u0131n ortaya \u00e7\u0131k\u0131\u015f\u0131n\u0131, etkisi b\u00fcy\u00fck ve g\u00f6rece k\u0131sa s\u00fcre i\u00e7inde (a\u015f\u0131r\u0131 durumunda, sadece bir ku\u015fak sonras\u0131nda) ortaya \u00e7\u0131kan mutasyonlara ba\u011flamas\u0131yla art\u0131k geride kalm\u0131\u015f \u201cmutasyonculuk\u201d g\u00f6r\u00fc\u015f\u00fcne ba\u015fvuran bu yakla\u015f\u0131m\u0131n s\u0131nanmas\u0131nda Limulus<\/em>\u2019un pay\u0131 b\u00fcy\u00fck olmu\u015ftur.<\/p>\n

\"\"
Polonya\u2019daki Kcynia Formasyonu\u2019nda saptanan fosil t\u00fcr Limulus darwini, Jura d\u00f6neminin en \u00fcst tabakas\u0131na ya\u015fland\u0131r\u0131lm\u0131\u015ft\u0131r (yakla\u015f\u0131k 148 milyon y\u0131l \u00f6ncesi) ve ya\u015fayan at nal\u0131 yengeci t\u00fcr\u00fc Limulus polyphemus\u2019a morfolojik a\u00e7\u0131dan neredeyse ikizi kadar benzemektedir.<\/figcaption><\/figure>\n

Robert Selander ve meslekta\u015flar\u0131, 1970 y\u0131l\u0131nda yay\u0131nlad\u0131klar\u0131 bir \u00e7al\u0131\u015fmada Limulus<\/em>\u2019un genetik \u00e7e\u015fitlilik d\u00fczeyini ele ald\u0131lar ve o g\u00fcnden bu g\u00fcne dek genomik d\u00fczeyde de do\u011frulanan \u00e7arp\u0131c\u0131 sonu\u00e7lara ula\u015ft\u0131lar.2<\/sup> Selander ve meslekta\u015flar\u0131, genetik \u00e7e\u015fitlili\u011fi tahminlemede o d\u00f6nemlerde yayg\u0131n bi\u00e7imde kullan\u0131lan protein elektroforezini kullanarak, Limulus<\/em>\u2019ta genetik \u00e7e\u015fitlilik g\u00f6steren \u00f6nemli say\u0131da gen oldu\u011funu saptad\u0131lar. \u00d6te yandan genetik \u00e7e\u015fitlilik g\u00f6stergesi olarak pop\u00fclasyon geneti\u011finde ba\u015fvurulan iki \u00f6nemli parametreye (polimorfik lokus oran\u0131 (P) ve bireysel heterozigotluk (H)) g\u00f6re, bu \u00e7e\u015fitlilik miktar\u0131n\u0131n Drosophila, fare, insan gibi farkl\u0131 t\u00fcrlerdeki d\u00fczeylere \u00f6nemli \u00f6l\u00e7\u00fcde benzer oldu\u011funu g\u00f6sterdiler. Yaln\u0131zca bir avu\u00e7 proteini kodlayan az say\u0131daki gene odaklanm\u0131\u015f olan bu \u00f6nc\u00fc \u00e7al\u0131\u015fman\u0131n sonu\u00e7lar\u0131, yak\u0131n tarihli genom d\u00fczeyli \u00e7al\u0131\u015fmalarla da do\u011frulanm\u0131\u015ft\u0131r.3,4<\/sup><\/p>\n

Neden bu denli de\u011fi\u015fmemi\u015f durumdalar?<\/strong><\/p>\n

G\u00f6rece k\u0131sa bir s\u00fcre \u00f6nce de, Limulus polypehmus<\/em>\u2019un fosil bir akrabas\u0131 olan Limulus darwini<\/em> de bilim d\u00fcnyas\u0131n kazand\u0131r\u0131lm\u0131\u015ft\u0131r: Polonya\u2019daki Kcynia Formasyonu\u2019nda saptanan bu fosil t\u00fcr, Jura d\u00f6neminin en \u00fcst tabakas\u0131na ya\u015fland\u0131r\u0131lm\u0131\u015ft\u0131r (yakla\u015f\u0131k 148 milyon y\u0131l \u00f6ncesi) ve ya\u015fayan at nal\u0131 yengeci t\u00fcr\u00fc Limulus polyphemus<\/em>\u2019a morfolojik a\u00e7\u0131dan neredeyse bir ikizi kadar benzemektedir.5<\/sup><\/p>\n

\u015eimdi, modern evrimsel genetik ve genomik \u00e7al\u0131\u015fmalar\u0131n, g\u00f6rece h\u0131zl\u0131 evrimle\u015fen di\u011fer t\u00fcrler kadar genetik \u00e7e\u015fitlili\u011fe sahip oldu\u011funu g\u00f6sterdikleri at nal\u0131 yengeci, 150 milyon y\u0131l kadar \u00f6nce ya\u015fad\u0131\u011f\u0131 d\u00fc\u015f\u00fcn\u00fclen akraba t\u00fcr\u00fcne g\u00f6re bile neden bu denli de\u011fi\u015fmemi\u015f bir durumdad\u0131r? Bir ba\u015fka deyi\u015fle, biri art\u0131k ortadan kalkm\u0131\u015f olsa da (L<\/em>. darwini<\/em>) iki farkl\u0131 t\u00fcr neden bu denli benzer ve tarih boyunca de\u011fi\u015fim ge\u00e7irmemi\u015f g\u00f6z\u00fckmektedir? L. darwini<\/em>\u2019yi bulan ara\u015ft\u0131r\u0131c\u0131lar, Limulus, Coelacanth ve di\u011fer \u201cya\u015fayan fosiller\u201din ya\u015fam\u0131n tarihinin \u00e7ok uzun d\u00f6nemleri boyunca de\u011fi\u015fmeden kalm\u0131\u015f g\u00f6z\u00fckmelerini, yery\u00fcz\u00fcn\u00fcn be\u015f b\u00fcy\u00fck kitlesel yok olu\u015fundan en az birinden sa\u011f \u00e7\u0131kan formlar\u0131n, y\u00fcksek adaptasyon g\u00f6stermeleri ve d\u00fc\u015f\u00fck ekolojik de\u011fi\u015fkenlik g\u00f6steren ortamlarda ya\u015fayagelmeleriyle izah ediyorlar ve bu y\u00fcksek adaptasyon-d\u00fc\u015f\u00fck de\u011fi\u015fim ili\u015fkisini ortaya koymak i\u00e7in de \u201cstabilomorf\u201d terimini \u00f6neriyorlar. Bununla birlikte, biraz fazla geni\u015f bir tan\u0131msal aral\u0131kta evrimsel kararl\u0131l\u0131\u011f\u0131 ifade etmesine kar\u015f\u0131n makul bir \u00e7er\u00e7eve sunmaya aday olan bu yakla\u015f\u0131m, standart genetik \u00e7e\u015fitlilik seviyesine sahip olunmas\u0131na kar\u015f\u0131n neden bu genetik \u00e7e\u015fitlili\u011fin morfolojik-anatomik bir \u00e7e\u015fitlilik olarak ifade edilmedi\u011fini (yani bu d\u00fc\u015f\u00fck de\u011fi\u015fim seviyesini) a\u00e7\u0131klam\u0131yor.<\/p>\n

\"\"
Kuzey Amerika sahillerinde at nal\u0131 yenge\u00e7leri (Limulus polyphemus).<\/figcaption><\/figure>\n

\u00d6n\u00fcm\u00fczdeki ay, sadece Coelacanth ya da Limulus gibi d\u00fc\u015f\u00fck evrimsel de\u011fi\u015fim g\u00f6steren canl\u0131lar\u0131n bu durumlar\u0131n\u0131 a\u00e7\u0131klamakla kalmay\u0131p, genler ile fenotipler aras\u0131ndaki ili\u015fkinin indirgemeci yorumunun ciddi bir ele\u015ftirisine de imk\u00e2n tan\u0131yan ve b\u00f6ylece sa\u011fl\u0131kl\u0131 bir evrimsel biyoloji ve genetik icras\u0131n\u0131 m\u00fcmk\u00fcn k\u0131lmada anahtar rol\u00fcne sahip olan bir kavrama, gizli genetik \u00e7e\u015fitlili\u011fe de\u011finece\u011fiz. Gizli genetik \u00e7e\u015fitlili\u011fin neden gizli kalm\u0131\u015f oldu\u011fu, hangi ko\u015fullar alt\u0131nda bu gizli \u00e7e\u015fitlili\u011fin fenotipik \u00e7e\u015fitlilik ve evrimle\u015fme ba\u011flam\u0131nda a\u00e7\u0131\u011fa \u00e7\u0131kabildi\u011fi bahsine girece\u011fiz.<\/p>\n

Kaynaklar<\/strong><\/p>\n

1) Nikaido ve ark. (2013). Coelacanth genomes reveal signatures for evolutionary transition from water to land, Genome Research<\/em>, 23: 1740-1748.<\/p>\n

2) Selander, R.K., Yang, S.Y., Lewontin, R.C. ve Johnson, W.E. (1970). Genetic variation in the horseshoe crab (Limulus polyphemus), <\/em>a phylogenetic “relic”, Evolution<\/em>, 24: 402-414.<\/p>\n

3) Nossa, C.W. ve ark (2014). Joint assembly and genetic mapping of the Atlantic horseshoe crab genome reveals ancient whole genome duplication, GigaScience<\/em>, 3: 9.<\/p>\n

4) Simson, S.D. ve ark.(2017). The draft genome and transcriptome of the Atlantic horseshoe crab, Limulus polyphemus<\/em>, International Journal of Genomics<\/em>, http:\/\/dx.doi.org\/10.1155\/2017\/7636513.<\/p>\n

5) Kin,A. ve Blazejowski,B. (2014). The horseshoe crab of the genus Limulus: Living fossil or stabilomorph?, PLOS ONE<\/em>, 9(10), e108036.<\/p>\n","protected":false},"excerpt":{"rendered":"

Ara\u015ft\u0131r\u0131c\u0131lar, Limulus, Coelacanth ve di\u011fer \u201cya\u015fayan fosiller\u201din ya\u015fam\u0131n tarihinin \u00e7ok uzun d\u00f6nemleri boyunca de\u011fi\u015fmeden kalm\u0131\u015f g\u00f6z\u00fckmelerini, yery\u00fcz\u00fcn\u00fcn be\u015f b\u00fcy\u00fck kitlesel yok olu\u015fundan en az birinden sa\u011f \u00e7\u0131kan formlar\u0131n, y\u00fcksek adaptasyon g\u00f6stermeleri ve d\u00fc\u015f\u00fck ekolojik de\u011fi\u015fkenlik g\u00f6steren ortamlarda ya\u015fayagelmeleriyle izah ediyorlar. Ge\u00e7ti\u011fimiz ayki yaz\u0131m\u0131zda, \u201cya\u015fayan fosil\u201d olarak tabir edilen Coelecanth\u2019\u0131n, fosil formu ile ya\u015fayan t\u00fcrleri aras\u0131ndaki […]<\/p>\n","protected":false},"author":67,"featured_media":18193,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[2146,1475,21,32],"tags":[1848,200,2205,2204],"class_list":["post-18192","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-166-sayi","category-bilim-ve-tarih","category-biyoloji","category-evrim","tag-canlilik-tarihi","tag-evrim","tag-morfoloji","tag-yasayan-fosiller"],"acf":[],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t