Are there bhikkhunis in Theravada tradition?

This question cannot be answered in a simple "yes" or "no." We need to understand that when the Buddha established four groups of Buddhists, namely bhikkhus, bhikkhunis, laymen and laywomen, there was no distinction between Theravada and Mahayana. In fact the differentiation came into existence many hundred years afterwards.

The bhikkhunis who went to plant the seed of ordination in China belonged to Theravada, and even the vinaya which the Chinese Sangha follow is Dhammagupta, subsect from Theravada.

During King Asoka's period in the 3rd century B.E. there were at least 32 schools but with clear record and separate set of teachings only 18 schools were established, twelve sprang from the early branch and eight emerged from the Mahasanghika which could be roughly said to be the forerunner of Mahayana.

The basic reason for Theravada not to accept bhikkhuni Sangha tracing their lineage from Mahayana lineage is unfounded. The ordination lineage followed by Mahayana derived from early Buddhism.

Next question is how does Mahayana differ from Theravada? Generally speaking Mahayana differs from Theravada in its philosophical exposition of Dharma. However, the highly complex way of explaining dharma all took root from the early teaching of Buddhism which branched out and blossomed in Mahayana.