Abstract
Dinoflagellates of the Symbiodiniaceae family encompass diverse symbionts that are critical to corals and other species living in coral reefs. It is well known that sexual reproduction enhances adaptive evolution in changing environments. Although genes related to meiotic functions were reported in Symbiodiniaceae, cytological evidence of meiosis and fertilisation are however yet to be observed in these taxa. Using transcriptome and genome data from 21 Symbiodiniaceae isolates, we studied genes that encode proteins associated with distinct stages of meiosis and syngamy. We report the absence of genes that encode main components of the synaptonemal complex (SC), a protein structure that mediates homologous chromosomal pairing and class I crossovers. This result suggests an independent loss of canonical SCs in the alveolates, that also includes the SC-lacking ciliates. We hypothesise that this loss was due in part to permanently condensed chromosomes and repeat-rich sequences in Symbiodiniaceae (and other dinoflagellates) which favoured the SC-independent class II crossover pathway. Our results reveal novel insights into evolution of the meiotic molecular machinery in the ecologically important Symbiodiniaceae and in other eukaryotes.
Highlights
Dinoflagellates of the Symbiodiniaceae family encompass diverse symbionts that are critical to corals and other species living in coral reefs
Postmeiotic segregation increased homologs 1 and 2 (PMS1 and PMS2) are part of the DNA mismatch repair system during both mitosis and meiosis. They compete to heterodimerise with MutL homolog 1 (MLH1), which is assembled into the MutL-MutS heteroduplex that aids in degradation of DNA strands
We found that meiosis-specific genes involved in the formation of the synaptonemal complex (SC) were largely missing from these microalgae
Summary
Dinoflagellates of the Symbiodiniaceae family encompass diverse symbionts that are critical to corals and other species living in coral reefs. We report the absence of genes that encode main components of the synaptonemal complex (SC), a protein structure that mediates homologous chromosomal pairing and class I crossovers This result suggests an independent loss of canonical SCs in the alveolates, that includes the SC-lacking ciliates. Formation of the SC is triggered by the synapsis initiation complex proteins, more commonly known as the ZMM proteins: (a) the transversal filament protein Zip[1] (not to be confused with zinc transporter Zip1) attaches the central element to the pair of lateral elements; (b) Zip[2], Zip[3], and Zip[4] mediate protein-protein interaction; (c) Mer[3], a DNA helicase unwinds double-stranded DNA; and (d) the Msh4-Msh[5] heterodimer binds to Holliday junction[17]. GEX1 gene products are nuclear envelope proteins involved in karyogamy[19]
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