Outward-looking y/Ø alternations in Luganda

In a number of Eastern Bantu languages an issue of “outward looking morphology” arises from the interaction of the verb stem (root + suffixes) and what precedes it. Vowel-initial verb roots such as -er- ‘sweep’ appear as such when preceded by a CV- prefix in Luganda (Hyman & Katamba 1999: 371): (1) a. tú-er-a [tw-éer-a] ‘we sweep’, b. a-ér-a [a-yér-a] ‘s/he sweeps’, c. er-á [yer-á] ‘sweep!’. When preceded by a V- prefix (1b), or when initial (1c), a root-initial [y] appears. As seen in (2a), only V-initial roots acquire a [y], while (2b) shows that the root allomorphy is determined exclusively on the basis of the immediately preceding prefix: (2) a. tú-a-gu-a [tw-áá-gw-a] ‘we fell’, b. tú-a-er-a [tw-áá-yer-a] ‘we swept’. Hyman & Katamba (1999) show that the y- vs. Ø initial must be first established in order to assign the stem tones correctly; however, the presence vs. absence of y- cannot be known without “looking ahead” to see what will precede the stem—an “outward-looking” morphological effect, prohibited by Carstairs’ (1987) peripherality condition, an issue in both constructional and distributed morphology (Babolijk 2000, Embick 2010, Inkelas 2014, Svenonius 2014). We argue for generating two stems in parallel, one with initial y-, the other without, the correct one selected on the basis of strict locality with the preceding prefix or left edge. The basic motivating constraint is that a stem-initial vowel must have an onset in the output, either from a preceding CV- prefix or the y-.