In a hypothetical organism, at a four-celled embryonic stage, blastomere ‘X’ instructs one of the daughter cells of an adjoining blastomere ‘Y’ to take the fate ‘Ya’. The other daughter cell takes the fate ‘Yp’. This is illustrated in the figure below as lineages for Ya and Yp. If the X blastomere is removed, both daughter cells take up the Yp fate.

This instruction is mediated by a paracrine factor, Pap2 secreted by X blastomere interacting with Pap5 present on the membrane of Y blastomere.

The following experimental manipulations were carried out, which involved creating partial genetically mosaic embryos in vitro and following the fate of the Y blastomere.

Which one of the mosaics will show a developmental pattern similar to that when blastomere X is removed?

Correct option is A

In the normal developmental pattern, blastomere X secretes Pap2, which interacts with Pap5 on the Y blastomere. This interaction directs one daughter cell of Y to adopt the Ya fate, while the other adopts the Yp fate.

When blastomere X is removed, both daughter cells of Y take on the Yp fate because the instructive signal (Pap2) is absent.

To experimentally mimic this scenario:

• The absence of Pap2 in X would prevent signaling to Y, leading both daughter cells to assume the Yp fate.
• In option (1), X blastomere is null for Pap2, meaning it cannot secrete Pap2, effectively making it non-functional, just as if it were removed.
• Since the Y blastomere remains wild-type, it retains Pap5 but cannot receive the Pap2 signal due to its absence.
• Consequently, the fate of Y’s daughter cells will resemble the Yp/Yp pattern observed when X is removed.

Information Booster

1. Paracrine signaling involves molecules secreted by one cell to influence neighboring cells.
2. Pap2 acts as a signaling molecule released by X to regulate fate determination in Y.
3. Pap5 is the receptor protein on Y that interacts with Pap2 to induce the Ya fate.
4. Loss of Pap2 prevents differentiation into Ya, resulting in both daughter cells becoming Yp.
5. Mosaic embryos help study gene function by selectively disabling specific genes in certain cells.
6. Null mutation refers to a complete loss-of-function mutation in a gene, preventing the production of its protein.
7. Constitutive activation means that a receptor or protein remains active regardless of external signals.