A protocol for a single-cell PCR of diatoms from fixed samples: method validation using Ditylum brightwellii (T. West) Grunow

Abstract

Many major advances in macro-organismal biology, population genetics and evolutionary biology may be attributed to the development of rapid and inexpensive molecular methodology. Here, a simple protocol is presented for a polymerase chain reaction (PCR) of alcohol-preserved single diatom cells that facilitate studies requiring a large number of individuals, inclusive of archived and uncultivable specimens. Our approach integrates cell isolation and PCR amplification of two molecular markers, as well as post-PCR identification by scanning electron microscopy, thereby bridging the gap between classical and molecular diatom biology. The amplification of two genes was performed in a two-step protocol. By applying multiple sets of PCR primers to a single diatom cell, an internal fragment of the large subunit of the ribulose-1,5-bisphosphate carboxylase gene (RuBisCO, rbcL) was successfully amplified and sequenced, as well as the internal transcribed spacer (ITS) region in 20% of the same, preserved cells derived from natural phytoplankton. Multiple-gene recovery from a single-cell source requires further optimization, likely specific for the taxa of interest. Single marker recovery (from first-step amplification to sequencing) from natural, fixed single-cell samples was more successful with overall success rate of ∼31% for ITS and 67% for rbcL, respectively. Admittedly it is 50–70% lower than rates for a culture-pelleted DNA source (68% for ITS and 100% for rbcL) using current standard methods of sequence recovery in diatom studies, but our method is much less labor- and cost-intensive. This approach was applied to the diatom Ditylum brightwellii to assess the quality of recovered sequences. The phred quality scores of our sequences derived from single-cell material were similar to phred scores of our own, published sequences obtained from cultures grown in our laboratory.