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Fig. 4 Epistatic interactions of mdka. (a,c) In floating head mutants (flh-/-) that lack a notochord and only retain islands of MFP cells (a), mdka overepression partially rescues the MFP defect. (a,b) Cell number is increased, resulting in an almost continuous line one cell in diameter (n = 21/27, 78%). Knockdown has no effect on MFP cell number and does not rescue the notochord (n = 42, shh [c]; n = 18, ntl [data not shown]). (d-k) Delta/Notch-deficient mindbomb mutants (mib-/-) lack MFP cells (f) and were identified by an increase in isl1-positive neurons (ne, in red [g,i,k]). (h-k) Mdka overexpression and knockdown produces identical MFP and notochord phenotypes in mib-/- as in wild-type embryos. Overexpression enlarges the MFP (n = 11/15, 73% [h,i]) and knockdown results in MFP gaps larger than in noninjected mib-/- (n = 11/18, 61% [j]). (l-n) one-eyed pinhead (oep-/-) embryos that lack the entire MFP (m) were rescued by injection of mdka RNA (n = 12/14, 86% [n]). In these embryos, the broadness of the MFP was increased. oep-/- were identified by remaining gaps in the f-spondin-positive MFP (arrowhead). (o,p) Overexpression of cyclops (cyc) RNA in wild-type embryos resulted in an enlargement of the MFP from one to three cells in width (n = 36/57, 63% [o]). Coinjection of cyc RNA and mdka morpholino resulted in a MFP with nearly wild-type morphology (n = 73/87, 83% [p]). (q-v) In no-tail mutants (ntl-/-), MFP width is enlarged to variable extents (q,r,u), but is nearly rescued after mdka morpholino injection (s,v). ntl-/- embryos were identified by the absence of a notochord and oep expression (q-s,u,v). mdka knock-down is not able to rescue the notochord. Anterior is to the left in lateral (a-c,d,f,h,j) and dorsal (l-s) views; arrowheads indicate gaps in MFP. (e,g,i,k,t-v) Transverse sections at trunk level. Bars: c, 100 µm; p,s, 40 µm; j,k,t, 20 µm.