The role of maternal tissue in the control of embryogenesis remains enigmatic in many complex organisms. Here, we investigate the contribution of maternal tissue to apical-basal patterning in the kelp embryo. Using a modified kelp fertilisation protocol which yields synchronously developing kelp embryos, we show that detachment from maternal tissue leads to compromised robustness of apical-basal patterning. Detached embryos are rounder and often show aberrant morphologies. Furthermore, absence of contact with maternal tissue increases parthenogenesis, highlighting the critical role of maternal signals in the initial stages of kelp development. When zygotes are detached from the female gametophyte while part of the oogonial cell wall still remains attached to the egg, the proper apical-basal patterning is rescued showing a key role for the connection to the maternal cell wall in developmental patterning in kelps. This observation is reminiscent of another brown alga, Fucus, where the cell wall has been shown to play a key role in cell fate determination. In the case of kelps, the maternal oogonium mediates basal cell fate determination by providing an extrinsic patterning cue in its extracellular matrix to the future embryo. Our findings suggest a conserved mechanism across phylogenetically distant oogamous brown algal lineages, where localised secretion of sulphated F2 fucans mediate establishment of the apical-basal polarity.

Cell wall-mediated maternal control of apical-basal patterning of the kelp Undaria pinnatifida