COULD there be a by-election in Derek Mackay’s Renfrewshire North and West constituency? Well, that depends, of course, on what the former finance secretary does or is forced to do in terms of his status as an MSP.

Mackay dramatically resigned as a key member of the Scottish Government on Thursday after it emerged he had been messaging a 16-year-old boy. He has been suspended from the SNP pending an investigation into his actions and is currently an independent MSP. It is possible the politician could be re-admitted into the party in which case he would return as an SNP MSP.

However, if not, events could unfold in at least two possible ways.

One option is that he stays on as an independent MSP until the Holyrood election in May 2021. He might choose to fight that election as an independent.

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Mark McDonald, the former minster for early years and Aberdeen Donside MSP, currently sits as an independent after he resigned from the SNP in March 2018 over inappropriate behaviour.

He refused to stand down from Holyrood, despite senior colleagues, including Nicola Sturgeon, expressing their dismay at his decision. The Commissioner for Ethical Standards in Public Life concluded that McDonald’s behaviour amounted to sexual harassment, and he was later suspended from Parliament for a month without pay following an inquiry by Holyrood’s standards committee.

Mackay, if he is not re-admitted to the SNP, might not wish to continue as an independent MSP and could step down well ahead of the election next May.

Such a decision would prompt a by-election.

Unlike the Holyrood election where the timing is decided by law and take place every four years, normally on the first Thursday of May, the date of a by-election is set by the Scottish Parliament’s Presiding Officer, under Section 9 of the Scotland Act 1998.

When fixing the date, the Presiding Officer does the following: consults the relevant returning Officer (RO) – the RO will highlight any difficulties foreseen in terms of timing or fulfilling other legal requirements; and take soundings from each of the main parties on the possible date of the poll starting.

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The Presiding Officer will recognise the protocol that the party which held the seat’s view has greater weight in the decision-making process, but he will be free to exercise his discretion as he sees fit.

The Presiding Officer then takes a final decision on the date and communicates that to the returning officer and to Parliament.

An SNP by-election candidate is selected by the party headquarters rather than the local branch. Last night politicians were tight-lipped about how they sensed events could unfold and who possible candidates may be. An obvious contender would be Mhairi Black, 25, the Paisley and Renfrewshire South MP recently promoted by Ian Blackford as shadow Scottish secretary. Her addition to the SNP’s Holyrood team would bring in new talent into the group and if given a ministerial role help refreshen the government’s image.