  The first step toward planet formation is grain growth from (sub-)micrometer
to millimeter/centimeter sizes. Grain growth has been reported not only in
Class II protoplanetary disks but also in Class 0/I protostellar envelopes.
However, early-stage grain growth occurring in Class 0/I stages has rarely been
observed on the protostellar disk scale. Here we present the results from the
ALMA Band 3 ($\lambda$ = 3.1 mm) and 7 ($\lambda$ = 0.87 mm) archival data of
the Class I protostellar disk WL 17 in the $\rho$ Ophiuchus molecular cloud.
Disk substructures are found in both bands, but they are different: while a
central hole and a symmetric ring appear in Band 3, an off-center hole and an
asymmetric ring are shown in Band 7. Furthermore, we obtain an asymmetric
spectral index map with a low mean value of $\alpha$ = 2.28 $\pm$ 0.02,
suggestive of grain growth and dust segregation on the protostellar disk scale.
Our radiative transfer modeling verifies these two features by demonstrating
that 10 cm-sized large grains are symmetrically distributed, whereas 10
$\mu$m-sized small grains are asymmetrically distributed. Also, the analysis
shows that the disk is expected to be massive and gravitationally unstable. We
thus suggest a single Jupiter-mass protoplanet formed by gravitational
instability as the origin of the ring-like structure, grain growth, and dust
segregation identified in WL 17.
