Question

Chloroplasts of certain algal taxa such as Euglenophyta are surrounded by three membranes while, Heterokontophyta and Cryptophyta have four membranes. The chloroplasts present in these organisms is a result of secondary endosymbiotic events. All of them have chlorophyll a. However, Euglenophyta uses chlorophyll b as an accessory pigment, whereas Heterokontophyta and Cryptophyta use chlorophyll c. Assuming gain-of-function and gene addition, the most likely evolutionary scenario best describing the origins of chloroplast in these organisms is:

• A. While Heterokontophyta and Cryptophyta had a common secondary endosymbiont, Euglenophyta evolved from an independent secondary endosymbiont.
• B. Heterokontophyta, Cryptophyta and Euglenophyta all had a common secondary endosymbiont.
• C. Heterokontophyta, Cryptophyta and Euglenophyta all evolved from independent secondary endosymbiotic events.
• D. Heterokontophyta and Cryptophyta were a result of secondary endosymbiotic event from Euglenophyta.

Answer 1

Answer: (A)

While Heterokontophyta and Cryptophyta had a common secondary endosymbiont, Euglenophyta evolved from an independent secondary endosymbiont.

Answer 2

Euglenophyta possesses chlorophyll b and 3 chloroplast membranes, indicating its chloroplast originated from a secondary endosymbiosis involving a green alga. Heterokontophyta and Cryptophyta both possess chlorophyll c and 4 chloroplast membranes, indicating their chloroplasts originated from a secondary endosymbiosis involving a red alga. The distinct accessory pigments (b vs. c) signify independent origins from different types of primary endosymbionts (green alga vs. red alga). The shared characteristics (chlorophyll c and 4 membranes) between Heterokontophyta and Cryptophyta strongly suggest a common secondary endosymbiotic event from a red alga. Therefore, Euglenophyta's chloroplasts arose independently, while Heterokontophyta and Cryptophyta share a common secondary endosymbiont.