Q49 - Heterostyly and Pollen Transfer Efficiency in Primrose

Theoretical A Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Flower Anatomy & Reproductive Structures Population Genetics & Hardy-Weinberg Equilibrium

Primrose has heterostyly, i.e. two flower forms on different individuals: P-plants with long style and short stamens inside the corollar tube, and T-plants with short style and stamens higher in the corolla (Fig. A). Darwin crossed T x P primroses (Table), and found that heterostyly increases outcrossing and thus fruit set. A bee inserts its tongue in a P-flower and gets pollen on the tip of its tongue, and then places the pollen on the stigma of a T-plant and vice versa for T-pollen to P-stigma (Fig. B). Heterostyly is controlled by one gene (T is Ss and P is ss; SS is non-viable, S is dominant to s). A, flower forms P (left) and T (right) of primrose; B, flowers P and T, with a bee’s tongue in between; arrows show amount of pollen transferred along different routes from the anther, to tongue and to the stigma. Fruit set (Number of fruits) after 100 T x T and P x P crossings and 100 T x P and P x T crossings.

Figure 1. Figure 1.

Using the information and data, determine which of the statements are true or which are false.

A. Frequencies of T and P in a primrose population stay the same, if mating is random with respect to form, and all crossings give the same fruit set

B. Incomplete sterility within the same flower form is seen in the population as a deviation from a 1: 1 T: P ratio

C. The two routes of pollen transfer (T --> P or P --> T) are equally efficient in terms of the pollen transferred

D. Pollen deposition from tongue to stigma is more difficult than pollen harvesting from anther to tongue

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Question reproduced from IBO 2015, Theoretical Exam A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)