If the earth were not tilted in its orbit how would the climates and seasons be affected at the equator and between 30 north and south latitudes

There would be one season (summer), and it would be always hot

Explanation:

What causes climates and seasons is the tilted orbit of Earth, this is caused by sunlights which come to Earth with different angles all over the Earth. The last palce to get sunlight with 90 degrees are 23 degrees 27 minutes N and S latitudes. When sunlight comes directly to the Earth longtitude between 30 N and S would get sunlights with 90 degrees all year.

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link: