Evolution of Ac and Dsl elements in select grasses (Poaceae)

Area of Science:

• Molecular Evolution
• Genetics
• Bioinformatics

Background:

• Transposable elements (TEs) like Ac/Ds are mobile genetic sequences crucial for genome evolution.
• Understanding TE evolution in grasses (Poaceae) provides insights into plant genome dynamics and diversification.
• Previous studies have characterized Ac/Ds elements in maize but their broader evolutionary history in grasses remains less understood.

Purpose of the Study:

• To investigate the evolutionary history of Ac-like transposable elements in pearl millet (Pennisetum glaucum) and compare it with maize Ac sequences.
• To analyze the evolutionary relationships and mutation patterns of Ds1 controlling elements in teosinte species.
• To infer the ancient origins and distribution of Ac/Ds family elements within the grass lineage.

Main Methods:

• DNA sequencing of an Ac-like element from pearl millet.
• Sequence alignment and comparative analysis of pearl millet and maize Ac sequences.
• Phylogenetic analysis using maximum likelihood networks for Ds1 elements from maize, teosinte, and Tripsacum.

Main Results:

• The pearl millet Ac-like sequence lacks intron II present in maize Ac, with an estimated evolutionary distance of 0.429 substitutions/site.
• This divergence rate is comparable to neutral evolution rates, suggesting Ac-like elements are not strongly constrained by selection.
• Ds1 elements from teosinte species are as divergent from maize Ds1 elements as maize Ds1 elements are from each other, indicating independent mutation accumulation.

Conclusions:

• Ac-like sequences likely existed in the common progenitor of maize and pearl millet, dating back at least 25 million years, and may be widespread in grasses.
• The data support the independent evolution and mutation accumulation of Ds1 elements since the divergence of maize and Tripsacum.
• These findings provide a framework for understanding the long-term evolutionary dynamics and origins of the Ac/Ds transposable element family in grasses.