Pre-meiotic endomitosis in the cytokinesis-defective tomato mutant pmcd1 generates tetraploid meiocytes and diploid gametes

Sexual polyploidization through the formation and functioning of 2n gametes is considered a major route for plant speciation and diversification. The cellular mechanism underlying 2n gamete formation mostly involves a restitution of the meiotic cell cycle, generating dyads and triads instead of tetrad meiotic end-products. As an alternative mechanism, the tomato mutant pmcd1 (for pre-meiotic cytokinesis defect 1), which generates diploid gametes through the ectopic induction of pre-meiotic endomitosis, is presented here. Using cytological approaches, it is demonstrated that male pmcd1 meiocyte initials exhibit clear alterations in cell cycle progression and cell plate formation, and consequently form syncytial cells that display different grades of cellular and/or nuclear fusion. In addition, it was found that other somatic tissue types (e.g. cotyledons and petals) also display occasional defects in cell wall formation and exhibit alterations in callose deposition, indicating that pmcd1 has a general defect in cell plate formation, most probably caused by alterations in callose biosynthesis. In a broader perspective, these findings demonstrate that defects in cytokinesis and cell plate formation may constitute a putative route for diplogamete formation and sexual polyploidization in plants.