A brief history of the Yemenite Etrog
The Jews of Yemen traditionally called the mitzvah of Arba’as Haminim “sh`ana”, derived from the word “hoshanah”.
The Yemenite etrog is distinct in that the mature fruits are generally larger than other etrog varieties. The Gemara in Masechet Sukkot 36b relates that “There was an incident involving Rabbi Akiva, who came to the synagogue, and his etrog was so large that he carried it on his shoulder”.
Citing this Gemara, Rabbi Yosef Kafih and Rabbi Yitzhak Ratzhabi, assert that only Yemenite etrogim can truly be said to have a chazaka of being grown from non-grafted trees, as they are the only variety that could “weigh as much as three kilograms”. They hold that some of the other etrog varieties may have been grafted with lemons (which are small), and this has stunted their capacity to grow. On this basis, they consider Yemenite etrogim to be more kosher and more mehudar than the other etrog varieties.
Other people point to the fact that Yemenite etrogim lacks pulp as an indication that the species remains ungrafted (unlike other varieties) and thus may be the closest to the "original" etrogim used by the Jews in days of old.
Yemenite etrogim are still grown in the orchards of Yemen, as well as by Jews of Yemenite ancestry in Eretz Yisroel.