Idiom for model organism / prototypical example / "MNIST of X"

Question

I'm looking for idioms or phrases conveying something similar to one of the following:

• a toy example on which practitioners of X usually test a new method first, as it's expected that if a method fails on this simple case, it would not work on more complex ones;
• a fundamental or well-understood example all practitioners of X are taught when getting introduced to some method;
• a stepping stone subject or task that is under study due to the perceived relative simplicity compared to other subjects or tasks of interest.

An example of a good answer would be "MNIST of X", as sometimes used in the machine learning literature, but I'm looking for something that sounds a bit less technical.

An example sentence: "Drosophila melanogaster is idiom of genetics". I'm not attached to this particular sentence structure.

Answer 1

MNIST is a dataset of handwritten digits and a popular test bed for image processing systems.
Drosophila melanogaster has been the test bed par excellence of classical genetics.

A testbed (also spelled test bed) is a platform for conducting rigorous, transparent, and replicable testing of scientific theories, computing tools, and new technologies.
Wikipedia

test bed (n.)

A vehicle (such as an airplane) used for testing new equipment (such as engines or weapons systems)

Broadly: any device, facility, or means for testing something in development

California also has a history of being a test bed for future federal U.S. policies.
— Lily Hsueh, Fortune, 10 Oct. 2023
M-W

test bed

[ANSI] An environment containing the hardware, instrumentation, simulators, software tools, and other support elements needed to conduct a test.
[LIS] Any system whose primary purpose is to provide a framework within which other systems can be tested. Test beds are usually tailored to a specific programming language and implementation technique, and often to a specific application. Typically a test bed provides some means of simulating the environment of the system under test, of test-data generation and presentation, and of recording test results.
"QA and Test Glossary" at xqual.com

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Image classification is a particularly attractive test-bed to evaluate deep learning networks. We have previously encountered the MNIST database, which consists of greyscale images of handwritten digits. This time, we will show how both unsupervised and supervised deep learning techniques can be used to learn from the same dataset.
Uday Kamath and Krishna Choppella; Mastering Java Machine Learning (2017)

In our testbed experiments we have used two datasets: the MNIST database of hand-written digits ... and the HHreco multi-stroke symbol database ...
Noel Lopes and Bernardete Ribeiro; Machine Learning for Adaptive Many-Core Machines (2015)

In 1965, Soviet mathematician Alexander Kronrod called chess "the drosophila of artificial intelligence." By that he meant that the game was to artificial intelligence research what the fruit fly had been to genetics research: a test bed for the field's biggest ideas, at once accessible enough to experiment on easily and complex enough to learn from.
Kartik Hosanagar; A Human's Guide to Machine Intelligence (2020)

On this basis, although yeast has proved to be a test-bed for comparative eukaryotic genomics, the nematode and fruit fly, albeit more resource intensive, hold greater promise.
L.J. Beeley et al.; "The Impact of Genomics on Drug Discovery" in F.D. King and A.W. Oxford (eds.); Progress in Medicinal Chemistry, Vol. 37 (2000)

From the seminal publication in 1996..., yeast served as the test bed for academic and commercial development of microarrayed DNA probes and was the first organism for which whole genome arrays were available.
Petra Ross-Macdonald; "Growing Yeast for Fun and Profit" in Pamela Carroll and Kevin Fitzgerald (eds); Model Organisms in Drug Discovery (2003)

As the second most frequent word not only in HistArt but also in written English more generally (Leech, Rayson & Wilson 2001:181), of constitutes an excellent test-bed for the claim that closed-class keywords are tractable to qualitative semantic analysis.
Nicholas Groom; "Closed-class keywords and corpus-driven discourse analysis" in Marina Bondi and Mike Scott (eds.); Keyness in Texts (1996)

Answer 2

How about archetype or canonical? Eg, "MNST is the archetypical dataset used in computer vision literature." Textbook example also works

Archetype: (1) a perfect or typical specimen (2) an original model or pattern; prototype. Collins English Dictionary

Answer 3

You may be looking for (an) exemplar.

Drosophila melanogaster is an exemplar of genetics (studies).

M-W:

exemplar noun
: one that serves as a model or example: such as
a) an ideal model
b) a typical or standard specimen
an exemplar of medieval architecture

Answer 4

(Collins) a case in point phrase
If you say that something is a case in point, you mean that it is a good example of something you have just mentioned.
• In many cases religious persecution is the cause of people fleeing their country. A case in point is colonial India.

(A Case in Point -- Lilly Moss, ‎Michael Quinn · 2017) . After much hemming and hawing, we finally came to an agreement that "A Case in Point" made more sense to the book. A case in point, meaning 'living proof' or 'working example' worked quite nicely.

Answer 5

As Yosef has said, pilot is appropriate:

pilot 3 of 3 [adjective] [I'd say attributive noun]:

serving as a guiding or tracing device, an activating or auxiliary unit, or a trial apparatus or operation

• a pilot study

[Merriam-Webster]

• This paper develops a discrete model of how pilot programs create value with a limited number of variables.

[Samis & Steen; Science Direct; 2020 ]

For a basic subject area under investigation, one could use

• the sine qua non of clinical trials

[Sanctum Law]

and for the actual specimen / theory being tested

• the sine qua non of test subjects

(see above reference for term 'test subjects').