PLANT BREEDING PROGRESS AND GENETİC DİVERSİTY FROM DE NOVO VARIATION AND ELEVATED EPISTASIS

Öz

Breeding programs in major crops normally restrict the use of parents to those improved for a variety of traits. Gain from utilising these good x good crosses appears to be high, and improvements are sufficient to encourage continued breeding within narrow gene pools even though each cycle is expected to lead to reduced genetic variability. These finely tuned programs have gradually limited the amount of new diversity introduced into the breeding gene pool. This breeding strategy has led to a genetic gap where there is a large difference in the favourable gene freguency between the improved and unimproved lines and to a narrovving of genetic diversity vvithin elite gene pools. At the same time, evidence has accumulated in plant breeding programs and long-term selection experiments in several organisms that the genome is more plastic and amenable to selection than previously assumed. in the barley (Hordeum vulgare L.) case study reported here, incremental genetic gains were made for several traits in what appears, based on pedigree analysis, to be a narrow gene pool. Given this situation, we call for an examination of the generally held belief that the variation on which selection is based in elite gene pools is provided almost exclusively from the original parents. Classical and molecular genetic analyses have shown that many mechanisms exist to generate variation de novo, such as gene amplification and transposable elements. Accordingly, we put forward the hypothesis that newly generated variation makes an important contribution.  We also hypothesize that gene interaction, epistasis, is more important than commonly viewed and that it arises from de novo generated diversity as well as the original diversity.

Anahtar Kelimeler

• Novo varitaion; plant breeding; gene interaction

BİTKİ ISLAHlNDAKİ BAŞARI VE ARTIRILMIŞ ÖRTÜCÜ GEN ETKİSİ İLE DE NOVO VARYASYONUNDAN SAĞLANAN GENETİK ÇEŞİTLİLİK

Öz

Yaygın yetiştirilen bitki türlerindeki ıslah programları birçok özellik yönünden geliştirilmiş ana babaların kullanımını sınırlamaktadır. İyi x iyi melezlerinin kullanımından sağlanan genetik ilerleme yüksek olarak görünmektedir ve her bir ıslah döngüsünün azalan genetik çeşitliliğe yol açacağı beklentisine rağmen gelişmeler bu dar genetik taban içinde bile ıslah çalışmalarına devam edilmesi için yeterince cesaret vericidir. Böyle iyi düzenlenmiş ıslah programları, gen havuzuna dahil edilen yeni genetik çeşitliliği kademeli olarak sınırlandırmıştır.

Bu ıslah yaklaşımı bir genetik farklılaşmaya (genetic gap) yol açmış geliştirilen ve geliştirilmemiş arzulanan gen frekansında büyük bir farklılık oluşmuş ve elit gen havuzundaki genetik çeşitlilikte daralmalar söz konusu olmuştur. Aynı zamanda birçok türdeki uzun süreli seleksiyon uygulamaları ve bitki ıslahı programlarındaki yeterli sayıdaki bulgulara dayalı olarak (seleksiyon için) genomun daha önce varsayılan daha esnek ve işletilebilir olduğu gözükmektedir. Burada pedigri seleksiyon yöntemi esas alınarak dar bir gen havuzuna rağmen arpada (Hordeum vulgare L.) birçok özellik yönünden önemli genetik ilerlemelerin sağlandığı bir durum saptama çalışması rapor edilmiştir. Bu durumda, genel olarak kabul gören bir uygulamaya ihtiyaç duyarız ki seleksiyonun elit gen havuzlarına dayalı olması durumunda varyasyon neredeyse yalnızca orijinal ana babalardan sağlanır.

Moleküler ve geleneksel genetik analizleri De Novo varyasyonu ortaya çıkaracak bir çok mekanizma olduğunu göstermiştir. Örneğin gen çoğaltımları (amplifikasyonları) ve yer değiştirebilir (transposable) öğeler gibi. Buna bağlı olarak, yeni oluşturulan varyasyonun bir önemli katkı yaptığı şeklindeki kuramı ileri sürebiliriz. Ayrıca geniş kabul gördüğünden öte gen etkileşimleri (interaksiyon) ve örtücü gen etkisinin (epistasi) daha önemli olduğunu ileri sürmek isteriz ve bu orijinal çeşitlilikten kaynaklandığı kadar De Novo kaynaklı çeşitlilikten de ortaya çıkmaktadır.

Anahtar Kelimeler

• Novo varyasyonu; bitki ıslahı; gen etkileşimi

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